on modern cartridge BBs, spindle length is measured end to end. in the old days of cup 'n' cone BBs, spindle length used to be measured end to end, and then center to end. you can still measure cartridge spindles this way, of course, but you can't really change them out, and they don't make a bunch of different 110mm BBs with varying drive-side spindle lengths, so it's pretty much pointless.