Having the right BB length is crucial to having gears that work as well as intended.

Middleburn cranks run a 48mm chain line for cross country and 50mm for downhill

For a standard MTB setup a 113mm axle is needed for ISIS or square taper.

However, there are far too many variables to always be able to predict what bottom bracket you need. BB-mount ("E-type") front derailleur and chain devices can throw a stick in the works, as can a lot of oddly designed frames.

If in doubt have your local bike shop fit your cranks.