The 0.7086" "addition" is measured perpendicular to the top surface and is parallel with the top surface.
1/4" (0.250") radius on the upper corners and on the front and back corners.
The cross drilling for the barrel nuts needs to be a size that makes the OD of the
barrel nuts a light press or heavy push fit in the fin head material.
No, the top of the Deep Tuttle drawing drawing suggests 0.724"/18.3896 mm from the front junction of the front taper and the top.... then 4.135"/105.029 mm between the hole centerlines.......0.491/12.4714 from the rear taper junction with the top to the center of the rear hole.
Spacing is the same as the std. Tuttle box (actually just extended up 18mm).
Centerline of the front barrel nut cross axis is 0.472/12.0 mm down from the top and the clearance drilling for the fin screws is 1.260/32 mm deep.
As far as where the hole in your fin box is, vs where Larry Tuttle meant for them to be
in the Tuttle fin head that's something you may need to alter.
I've used a rat tail (round) file to slot many boards to get the angle down to the barrel nuts right. Just use a large dia. fender washer with a good large dia. rubber washer under to seal the slot.
Several boards have had the fin screws coming out of the deck at pretty crazy angles.
Good engineering practice would have the fin screw holes counterbored concentric and
perpendicular to the clearance hole down to barrel nuts.
Hope this helps,