you sewed three 2.5 x 6 inch strips together and that made a 6 x 6 unit?
if that's what you did, you "lost" a half inch in the seam allowances - so at this point, your four-unit square should measure 11.5 inches
after those units are sewn together, they will "finish" at 11 inches
if all your units are ending up the same size, it should go together just fine
do you use commas or periods/dots when you separate the whole number from the decimal part of a number?
I get so confused when people say "finished" or "unfinished" size - to me, "finished" is the amount that would show after the blocks are sewn together - "unfinished" is the size of the block before it's put into the top
12.5 unfinished will be 12.0 inches "finished" size