So my question to you would be.... what are you making, and is there an easier way to make it?
Are you by any chance shooting for 2.5" finished HST's?
Because there are many other methods available where you don't need the 1/8" measurement, or ever have to deal with a bias edge, and you can still get perfect 3" unfinished HST's to use in your quilt block.
Just saying. ;) ;) ;) ;)
If you're not, then just ignore everything past this point. ;-)
Here's a quick and easy (with pictures) instruction guide on making the HST's:
http://quilting.about.com/od/quickpi...uaretria_2.htm
There are many similar guides out there.
The only thing I would change is the size of the starting squares.
If the instructions call for 3 3/8" squares (2.5" finished plus 7/8" seam allowance), I would start with 3.75" squares instead.
Make the HST the same way, AND then trim them down to the EXACT size I need - in your case 3" unfinished.
Yes, it's one extra step, but the extra little bit of fabric in the starting square will allow for any little woobles in the cutting or sewing of the HST's.