I find doing a float test with the commercial bait I want to imitate helps me figure out ballast size and placement. I make my knockoff, add the split rings and trebles, and then put it side by side with the original in my bucket of water, and adjust the ballast weight and location until my bait sits like the original. For me, it gives me a good starting point, and I can vary things from there, one at a time, to get my bait to work differently if I want to.