Hi John K.
Hmmmm.... I've never had to build one up on the front taper.
Did you look in the fin box with a flashlight to ensure there isn't already a lump of epoxy just below the surface on the back taper of the fin box?
I would rub some soap on the fin root and pull it down into the box and see where it's touching.
Should be a real tight "push" fit on the sides, and then the longer front taper should pretty much set
the angle of the fin.
I'm puzzled as to why the front would drop in lower than the rear.
Also have a look all the way down in the fin box and check for manufacturing debris that would prevent the back of the fin root from seating deeper in the box.
You should only be able to get a full contact on the front taper when the fin is at the correct angle and the root will be flush with the bottom of your board.
Can you send me some photos of your fin roots?
I may be able to check the angles to see that they are correct (with Larry Tuttle's original concept.
Hope this helps,