Setuping Up Prisms/Distance

I was just curious how most of u you guys setup your prisms to figure out the distance and angles before you permantly mounted them when using a ceiling mounted projector? I have tried stacking a number of boxes up and placing the prisms on top of those boxes but the boxes aren't perfectly level causing some issues.

Would I be able to take down the projector and put it on a table at same distances from the screen and then setup the prisms or will the math/angle be slightly different?

As always you guys are a great help, and thanks in advanced.

is the distance between the two prisms always the same? I was planning on mounting them with pivoting points, is the distance between these two pivot points a constant? regardless of the throw ratio ?