Tech Manual (corrected 4th printing) page 239 has masses for the different transport bays. Battle Armor bays weigh 2 tons per BA stored there, and require 6 personnel for each group of 4/5/6 BA based on the force type. I.e. Inner Sphere, Periphery, & Mercenary forces field its BA in units of 4 BA and each unit needs 6 personnel. The Clans field units of 5 BA, and need 6 personnel. Comstar/WoB field BA in units of 6 BA, and need 6 personnel.
You might choose to allocate extra tonnage, as having a 2-ton BA shoved inside a 2-ton bay means there is no room for anything else.
There are advanced rules that let you carry more of the smaller BA in the 2-ton bay, so if you want to make a more flexible force that could help lower the tonnage.
I.e. if you make a force that is primarily half-ton and 3/4 ton battle armor, then you probably want to carry more of the lighter BA per 2-ton bay.
