Yes, it feels silly writing 'ball park' all the time, but if I don't, someone always steps on my neck and reminds me that if someone pee's in the river upstream, the buoyancy will be affected.
Regarding the hard bait solution, I measure the volume of the body first, say it is 20g. This tells me that a suspended bait would be 20g. If I want 10% float, this means that the final weight of my bait must be 18g. I can weigh the bait with all the wires and hooks, allow a gram or two for paint and epoxy (varies on whether I decide to prime or not!!!) and know exactly how much ballast to add without playing submarines in the sink.
This is of course an over simplification, as when you add to the external surface by covering with epoxy and adding hardware, the volume increases. But, by keeping notes of volumes and weights, after half a dozen lures you get a good feel for the ballast required.
It all saves that sickening feeling as you watch your newest creation vanish below the surface. A bit like mixing too little epoxy, you only do it once.