The RULE is add 7/8" to the size you want, draw a diagonal line corner to corner, stitch 1/4" on each side of line then cut on line.
It is a tad easier to just add 1 " , mark, sew and cut then square up. You will only take a little sliver off --but-- it will be square. No actual waste just more accuracy. This is how I do it, but to each her own.
Have fun. Ruth