Take a 3/4" wood dowel and push down to FULL
contact with breech face. Mark the end of muzzle, withdraw and measure the mark to end of dowel.
That the proper way to measure that CRITICAL distance, determine where you need to cut and add 1/4". barrel do taper and this keeps you within area to make adjustments.
Use a dry-wall "T - square" to determine where you need to cut the stock for the 26" OAL.
This is what the ATFE uses, BUT are there STATE laws that are more stringent??
Check out before doing anything.