Butterflywing,
When you cut it a bit bigger and then sew wider seams, you run the risk of points not being where they need to be when you are finished, if you have a hard time seeing the measurments on your ruler, take some masking tape (I like the green or blue painters tape, it is low tac) and mark the line that you need to make your cuts. Then you only have to find it the once. some people use post it notes but I find I can knock them off of the ruler if I am not careful.