I believe the blanks are sized approximately 1" longer than their respective finished length. For example, that 29" heavy palma blank can be finished at 28" max, and a 27" blank is typically finished at 26".
Per their website, a Heavy Palma will be one inch overlength- 31"- to finish at 30" with a muzzle of of .90.
Taper after the step is only .004/inch in that contour, so cutting a few more inches off the barrel will increase muzzle OD by that amount.