I started wtih this

And wanted to add a little nub like this:

I managed to do it in the end, but I REALLY struggled and figured there must be an easy way to do it.

I started off by drawing a line which radiated from the point on the model I was trying to intersect:

And then I assuimed I would be able to draw a circle perpendicular to this line… But ever time I tried, it would end up off axis.

and

I know this is (or shouldbe) a reasonably trivial task, so there is clearly a tool or technique I am overlooking… How would I do this easily?