You’re right. There is something wrong, I just can’t find it. But I’ll keep trying. I like what you did there.
OK Box, I’m almost there. I can copy the profiles to all 4 corners, but when I go to erase the outside edges, the sides disappear. What did I miss?
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Sorry, I went to bed and it’s been a busy morning here.
The specific arc you want, a full quarter, doesn’t work with my method, I’m guessing because the last couple of vertices are too close together for sketchup to work with. Those same bits Dave referred to earlier. A tangential arc will work but won’t correctly represent what you are trying to model.
So @DaveR 's method is better.
Edit: And sure enough, by using The Dave Method, mentioned above, my version works too. So it is a tiny face issue at the point of the roundover.
It’s funny that is your answer. I pondered this all day yesterday. When I woke up I realized it was 90% likely it had to do with the precision of how I was drawing my arcs.
I was about to give up. but since both methods have value in learning how the software operates, it’s back to practice. Then I’ll be trying to roundover the bottoms 
I’m having a hard time understanding why this is so difficult when rounding over edges is such a common woodworking task. It seems to me SU should handle this better, and yes, easier.
For instance, before exploding the Curviloft render, close inspection shows correct geometry at the stopped end of the roundover. Explode it and the geometry goes bad. Now yesterday I did get it to work by using a scaled up copy, but this morning I can’t for the life of me recreate it. So I gave up and decided to just use what I had since it’s only a visual rendition of a roundover. Shouldn’t have to accept “good enough” in precision software.
My next step is to roundover the bottoms of the “legs”. I modeled a simple tool for that (in place on the leg Dave 
) but when I trim the leg weird stuff happens. This can be seen in the screenshot and the model.
I’ve seen Box and Dave do it so I know it can be done. My frustration lies in the difficulty of learning and understanding the many nuances in SU. I’ve also noticed some frustration from long time users when someone new such as myself can’t seem to get it right. It’s much more difficult to learn everything when you’re just starting out than to have learned new features as they’re released through the years.
A bit of a rant here and I apologize, but I am trying to figure this out. If I wasn’t so determined to not let something difficult defeat me, I would probably just give up and go back to pen and paper. So for now I persevere with the kind help of the members of this forum, SU’s Help and training videos.
Jeff
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Did you scale up again this morning? By how much? I opened the file I used for the example I did yesterday using the “Dave Method” and the geometry is correct. As I always do, I had scaled up by 1000.
If you are going to use Trim or SUbtract to do the rounding over, make a solid trimmer that represents the volume of the space the router bit would move though in a single pass around the leg. I use Follow Me for that. The round-cornered square is a copy of the bottom of the leg.
I used FredoCorner here although Round Corner would work as well. In either case no need to draw a cutter for this operation.
One thing I noticed in your file now and meant to mention yesterday is that you have Length Snapping enabled. This can get in the way of modeling precisely. Turn that off in Model Info>Units.
Thanks, I’ll try that for the bottom and work on the Dave Method again.
I guess that would be because SketchUp was made primarilly for architecture and not woodworking…
True, but it has been adopted by a fair population of the woodworking community as well, and as such, IMHO, it would be nice if there was some compatibility with that as well.
Even a simple task can be mathematically quite complex. Fillets and chamfers periodically run amok in other applications besides SketchUp too,
After some thought, I’m sure the coding end of this is quite involved and it’s simply a matter of the individual user deciding how much detail (and therefore practice/training) they need.
I’m understanding a little more. Me yesterday: I just got this new table saw but it takes forever to cut a dado! I have to move the fence 3/32" at a time like 20 times to just cut 1 3/4" dado! Why don’t they make it easy and just include a blade wide enough to cut it in one pass?
Most any power tool I use has accessories that can be bought/used to make it perform whatever task better. I use my dado set and shop made crosscut sled often. SU is much like any other tool I use. It’s pretty darn good at the basics and there are many tools available to make it even more useful for my purposes. So my apologies for complaining.
So I’m back at it this morning. I used RoundCorner to be able to see the effect of rounding over the bottom of the leg. It didn’t work like I know it should (some error on my part), but it’s enough that I can see the result.
Dave, I went back and enlarged copies of first cutter and leg part. I got the cutter right - geometry at the end of the stopped roundover is correct. But when I use it to cut the profile on the “leg” it leaves this hole in the leg. What did I miss? I’ve checked and I’m pretty sure the cutter is correctly aligned to the leg.
Forgot again.
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I’m not sure why it created a hole like that. I’ve never seen that happen before. Maybe the way you were holding your tongue?
It was easy enough fix by opening the component for editing and tracing one of the edges of the hole with the Line tool. I rotated your cutter around to the next corner on the leg and used Trim. No hole that time.
What tablesaw did you get?
LOL. That was a metaphor. I’m still using my 30 year old Delta.
Would you expect a beginner to be able to use your saw like you do in the first few weeks?
Oh no. It’s as if I told my 19 yo daughter to cut a board for me.
It is pretty simple right? I mean you just set the board under the miter saw blade, align the blade with the cut line and cut it. How hard is that? (said with tongue firmly in cheek.)
I just rotated the cutter and cut a second corner and it worked fine. I also traced a line of the hole to make the leg solid, but it left a square on the surface?
Good! I’m sticking with the way you were holding your tongue, then. 
Yes. Then get the Eraser tool, press Ctrl and click on those edges to soften them.
It’s important to understand how getting to grips with any software is very much like gaining experience with any tools.
Being drowned in information doesn’t help you gain a full understanding, you need to work through the process and find the answers as you go.
We could answer with ALL THE INFO but most of it would get lost in the noise.
Enjoy the learning process.