It depends on the boots, the bindings, and the position of the forward pressure when mounted. For example, I have two pairs of boots, both 27.5, one is a 310 BSL one is a 315 BSL, that's enough to cause me to adjust each time I switch boots. Bindings also make a difference, something like a Marker Griffon or a PX12 has a long track and can usually adjust + or - a few cms, while FKS bindings can only adjust about + or - about 5mms. Finally, where they are on the track makes a difference, say you have griffons, but to fit the 29.5s they're almost all the way forward on the track, you're not going to be able to adjust for those 28.5s.