I bought a 40" one-piece coated Dewey for my 26.5" barrel with muzzle brake
and Lucas bore guide. The way I figured out I needed a 40" was my 36" wouldn't clear the muzzle brake. :-)
Others may have their scientific methods. A 40" will be long enough for a 26" barrel, bore guide, and muzzle brake. And the extra couple inches won't hurt anything. Might even be long enough for a 28" barrel with brake and bore guide.