Are you saying I can't do it in my head or setup a proportion on paper if I wanted to? Are you saying I MUST support the royalties of the decendents of LT Philip Dalton?
Nope, I believe mental math is adequate so long as you get it right. Personally I did find use for it on the flip side for doing set/drift triangles. On the front side I did learn a nifty way to shift loads to meet MAC requirements when I did FEx years ago; I've forgotten since. Knowledge unused and all that...