Now you need the ports total thickness as measured from the widest/thickest point on the interior of the frame to the widest/thickest point on the exterior. I rounded this number to 34mm.
The math is now very easy. Simply add the interior and exterior offset numbers together. It looks like this:
13.5mm + 9mm = 22.5mm.
Next, simply subtract the offset totals of 22.5mm from 34mm. It looks like this:
34mm - 22.5mm = 12.5mm.
The cabin sides of our boat were roughly 12.5mm thick which translates to 0.4921 or roughly 1/2".
This is the number you call NFM with so they can calculate your total bolt length for you. It is very important to know this number before placing your order as it will save lots of time later. Physically cutting eight + bolts per port is a tedious and time consuming task! Trust me..
On a cabin with solid fiberglass sides, as my boat is, you will also want to order interior teak spacers or the spigot will protrude further than necessary. More on this bellow..