It's not silly, but I'm afraid it won't work. Let's say you had a 10" finished block made up of 5 squares across and 5 squares down. Each square would be 2" finished. If you added 2" to every measurement in the pattern, your squares would finish 4". That would give you a 20" block!
If you want to increase from 10" to 12", you need to increase each piece by 20% (12-10=2; 2 divided by 10=.20 or 20%). So your 2" square would now be 2.4". Add a .5" seam allowance all around, and you would cut your squares 2.9".
If you want to increase 8" to 12", you need to increase each piece by 50%, so a 2" square in the 8" block would finish 3" in a 12" block.
That's probably as clear as mud!