Well, I was going to rip on the "Troy" manual vs getting a real Honda manual (because that's usually the case), but I just checked a Honda manual and don't see that spec there either. It references page 15-2 for info on selecting the correct washer/spacer, but then doesn't have anything about that on page 15-2, nor any pages around that.
Its the drive gear that you're actually shimming, so that it correctly aligns with the vertical shaft bevel gear. Rather than buying the whole suite of washers, You'd want measure the existing one, do the assembly and measure freeplay between the two gears (how far the vertical shaft can rotate without moving the prop shaft) and adjust the shim as necessary. But without the spec that'd be impossible to do.
Interestingly, the BF8 manual says the shim spec is stamped inside the gear case and has a chart that shows which code relates to which shim is required.