I have done this kind of repair several times. There is no need to taper 12 to 1 - that is for a structural repair. Through hull
holes are not large enough to classify as structural.
I start by grinding the inside of the hull
and just grind the exposed edge of the hole enough to remove all caulk and dirt. A 45 degree angle outward is sufficient.
down one layer of mat and cloth on the inside. For these small holes it really doesn't matter what weight of cloth you use. Anything you have laying around it fine as long as it is clean and dry. I carry a bag of offcuts aboard. On the inside patch I overlap this about one inch onto the existing hull.
Then I grind out the outside back to the original fiberglass
edge again and take a little off the patch to remove any blush.
Then build up alternating layers of mat and cloth, four layers at a time, cleaning
the edge back to the original glass after each layup
. That way you are laminating onto the original hull and to the layer that you just laid down. Repeat until the full thickness of the hull is slightly exceeded.
Finally grind the patch fair to match the curve of the hull, seal with Interprotect 2000 or similar. The last time I did this there was someone in the yard doing a bottom job and he gave me a couple of brush fulls of it.
That's it. I have done four below the waterline and a three above over the last 30 years and none have failed. The only indication that they are there is the overlapping patch on the inside of the hull. I still own the same boat