DO you have two rectangular rulers? Lay the one along one edge of the block and butt the other one across the top of the block, butting the rulers together. You can see if your block is off. It takes a little more time, but you can get there.
If you have a piece of template plastic, you can use it also along with a standard ruler.
You can cut a square (the size you need) out of a piece of freezer paper or card board. Those can be used to check for size and then use the standard ruler to trim.