Not sure how this translates to foam lures but what I have done with wooden lures is this:
Prime lure and mount hardware (hooks and all). You want to be as close to the final weight distribution of the lure as possible to best determine where to place the weight.
Take the lure into the sink and using a finger on each side, find the spot where you can sink the lure while keeping it's back flat to the horizon. This may differ slightly from where you would balance the lure in the air since bouyancy differences of the materials and distribution of materials in the bait will make it float differently than it balances.
Mark this spot and drill out the hole for placing the lead. If you epoxy coat, this will make the bait slightly less bouyant so use slightly less lead than what it takes to get desired bouyancy at the primed stage.
When in doubt, favor more forward placement of lead vs back. This allows the bait to "back up" when you stop cranking which is handy for bouncing off structure.