Hello there Fred
Welcome to the Madhouse
okay
If you have a look at picture 6 on the build thread - you will see a red and a black arrow - both pointing at lines - drawn on the frame plan.
The red arrow is the outer deck line height. The black arrow, in actual fact, is the top of the rubbing strake which goes on the outside of the hull. So, the top measurement is taken from the line where the red arrow is pointing at.
Look at picture 7, you can see that this is of the 'keel' area and there are 2 more arrows pointing at lines here.
The red arrow is where the frame ends. The black arrow indicates the thickness/end of the external keel. So, the measurement of your frames are taken at where the red arrow is pointing at.
Now, if you have a look at photograph 1 which is the side view of the hull, look closely and just above the top of the rubbing strake you will see a dotted line, which follows roughly the shape of the hull - this is the height of your frames in the centre of the hull.
So it gives you to the top of the camber of the deck in other words.
Hope this is of some help.
If not - let me know and I will re describe what I have said as sometimes it is difficult to put into words (if you know what I mean).
Lets know if you need any further help. Don't forget as well, I have already reduced the size of frames by the thickness of planking - that thought just occurred to me.
John