So just to brush off my brain cells....
26/12 = 2.1667 so all your measurements would have to be multiplied by 2.1667 to give you a 26" block. Much easier to multiply by 2 and add that 1" sashing, right?
Or am I wrong about multiplying each measurement by 2.1667?
If you had a 26" block and wanted to shrink it down to 12" you would divide 12 by 26 to get your multiplier for the sides..... I think.