You know that the bow is higher than the transom in a sweeping fashion, so if you attach a string "line" to the front of the cap and the back of the cap you see the "sheer line" (if the console is out).
If you raise the line on the transom end, via a 1x2 or someway, a foot then you can start getting measurements.
Once you have the measurement you can draw out (very important for later) the depths from the line and then build supports to keep the sheer in the right sweep.