If not permanently attached, the muzzle brake does not need to be included in the length. If permanently attached, it does.
If you're of the opinion that the MA AWB applies to SBRs (IMO it does, but I'm not a lawyer), then you will need to permanently attach the muzzle brake, and thus should include its length.
If you're not sure what muzzle device you're going to use, just guess. When your build is complete, you can send a letter to the ATF notifying of changes to configuration (barrel length, overall length, etc.)