Well you have to remember that in general, lenses don't "age", you can have a lens through many projectors. Only thing I would consider now is if the lens you get will be "sufficient" for 4K. I wouldn't spend a lot of money on a lens today that I know I will want to upgrade when I get a 4K projector for example.
But getting back to ratios, I don't know, that's a tough thing. Balance is always good, but at the same time if you go "cheap" on a lens you may end up wanting to upgrade if you get a better projector down the line, vs getting a "good" lens up front and being able to use it through many projectors.