I've found, making a template or a partial template, out of paper, helps greatly. Helps if it's heavy paper and you have a roll of masking tape handy. This way you can sneak up on it trimming your paper instead of your final piece AND, if you cut too much paper away, you can re-adjust by using tape or taping paper back where you need it. And the tape helps hold your paper template in place. Extra hands, if you will.
