I would go with adding too much and drilling it out to adjust or getting it roughly correct but just under, and adding it with different weighted eyes or some lead wire around the front trebles
make your eye pupils out of lead and black nail polish and keep a range of different weights, that way you can finish the lure, float it in a glass of water/seawater, add weight till it sits the way you want it to, then select eyes to match the weight... you could even make floating eyes to add buoyancy.
-craig