Hmmm...lots of variety here in techniques.

Personally, I sink the float about half way. With a standard 5.5" dink, this takes about 2.5" of quarter inch pencil lead (held with tubing strung through the line).

If your looking to drop your jig 3-5' in medium flow, I don't think you need any weight other than the jig head. I rarely have a problem getting jigs down to the fish and I usually use 1/32 oz heads. In all but the lowest flows, surface currents are generally faster than currents at depth, so if you're just free spooling, the jig will run behind your float (no matter what the jig weight). Slow the float down some by thumbing your spool.

The ideal situation is what you described. You want the jig slightly ahead of your float. This automatically puts you jig head first, hook behind--when the fish bites, you hook the fish top jaw every time and he's not going anywhere unless the hook bends straight (which has been known to happen). If your losing a lot of fish, your problem may be what I described earlier--jig running behind the float with the head pointing at the fish.

I only add extra split shot or lengthen the leader substantially longer than bottom in faster currents, in which case I really lean on the float, thumbing my spool hard.

As far as light biters, again, leaning on your float and having your jig ahead of your float is the best way to get immediate response from the float (disappearing) when the fish bites. Only in very slow currents do I find that the float won't disappear with a bite. In these conditions, the float starts to dance on top.