Hi Sasafras!
The only time that 1/8" would really matter is if you are making a block as part of a group project. Then it's easier for the person sewing everybody's blocks together if they are all the same size.
We all have our own "PPM" - Personal Private Measurement - as Mary Ellen Hopkins says. As long as you're consistent, your blocks will fit together & if you decide it's smaller than you wanted to be at the end....add another border or another row or round of blocks!
Have fun!
sue