Pre-drilling before the dunk test is not the way to go. The Archimedes dunk test is all about the volume and by pre-drilling, you are reducing the volume. Stop thinking about the weight of wood that you are removing, it is clouding the issue. The dunk test gives you the volume of the lure in centimeter cubes. As one cc weighs 1 gram, then your target weight in grams is the same as the volume in cc's.
When you add a top coat you are increasing the volume so the pre-topcoat numbers no longer work, but you can allow for the top coat if you keep notes and weigh the bait before and after top coating. I guess I need to write a separate thread on how to deal with the top coat but I thought I had dealt with this issue.
Epoxy has a density of 1.2 approximately. 5/6ths of the weight goes to increasing the volume of the lure and 1/6th of the weight goes towards the ballast.
Example using Firechief's numbers:
Lure dry weight with hooks = 5 grams
Dunk test = 12 grams
Estimated epoxy weight = 1.6 grams
epoxy ballast = 1.6 / 6 = 0.27 grams
Ballasted lure to weigh 12 - 5 - 0.27 = 6.73 grams before epoxy.