I've found that borax overdrying is generally caused by borax that is extra fine(when it's more powdery more borax comes in contact with the eggs and causes more drying) or the fact that you're cutting the eggs into too small of pieces (the smaller the bait pieces the more borax will penetrate the eggs). I have the best results when I leave the eggs in larger chunks (3x2 inches) that I can cut down later to match the conditions. I also use a minimum of borax to surround the egg pieces.