Originally Posted by
floyd0117
Yes, that's exactly why.
If I know my chainline length (45mm), and I know my chamber width length, shouldn't I be able to precisely calculate my spindle length?
Nope, it mostly depends on the design of the crank arms. Years ago, crank arms were pretty much straight up and down and BB spindles were in the 120 mm's. More modern square taper crank arms are offset to the point that some require BB spindles less than 110 mm. You're right in thinking that it's OK to "cheat" by a few millimeters but it's a pain to guess wrong and find out that you bought a part that won't work.
My personal philosophy is, if I OWN parts I'll bolt almost anything up and see if I can adjust it to work to my satisfaction. If I'm BUYING parts I'll always hold out for stuff that I know is compatible.