A pop-up is a bait that is fished off the bottom and weighted down so that it is at a fixed Distance above the bottom of your choice. Normally overweighted in most cases.
Yet to critically balance a bait is so that the hook and Hookbait only just sinks, almost neutral buoyancy,behaving like a free bait. It can be a single bait or a multiple presentation.
Best way to do that is to decide if it is:
a) too buoyant, then add tiny pieces of putty to the rig or trim the pop-up, until it only just sinks.
b) too heavy, then trim the bottom bait(s) so that the pop-up (foam, plastic corn etc) does the same effect of the almost neutral buoyancy.
Does that help, or is anyone able to explain it better than that?