Take a pair of small battons, approx. 3/8 x 3/4 inch (long enough to overlap when in position) and with spring clamps attach one in position at the transom end and the other at the stem end. Mark on the battons a line somewhere where they overlap and measure the overall length of each to the line. Add the two and this is your length.
Now when you make the inwale cut it a 1/4 inch or so long so you can fit it up tight. Don't forget to allow for any angles and other features where it mates to the stem and transom.