there is a tool/ruler for this problem. i believe it is called add enough. i had the tool but can't find it right now. i used it when i first started paper piecing....i no longer need it.
just put in add enough ruler in the google or yahoo search.
hope this helps..don't give up it gets easier and FUN
oh yeah one more tip don't try to cut the piece to fit shape make it bigger and square it up.