Don’t do arithmetic! Just draw something temporary 10" from one end and pushpull the other end out until it snaps to that. Guides were made for this kind of thing!
There is no built-in way to display the three dimensions of an object, but there are various extensions that do so. Search the EW or sketchUcation and you will find them.
And if the geometry allows a push pull, it’s likely that a scale to the desired length will work too… (its difficult to answer this kind of question without seeing the object!)
I wold use Scale Tool the way Box describes. Then the part (beam?) can be a component scaled from the outside. By scaling the component itself and not PushPull the faces within it the same component definition can be re-used throughout the model which reduces the file size. It also allows you to at any point change the profile throughout the model if that is needed.