I would start small and low gravity wort. I'd go with 1.020 wort and 10x volume. So if you have 10ml of dregs add 100ml of low grav wort. You might want to decant and repeat that a couple times before moving to something larger. What I'll sometimes do is flame the bottle opening and add my low grav starter wort directly to the bottle with the dregs to about half full, and once it's done decant it off, and do it again but fill it up most of the way this time. Then I'll pitch that into a normal 1L 1.040 starter, see where I'm at and go from there.
As far as cell count, you're gonna either have to approximate or actually count them with a microscope (which would be the ideal). After a few steps, you can use a yeast harvesting slurry calculator to approximate what you actually have. You're starting with beat up yeast in small numbers, not a fresh pitchable from a lab, so you shouldn't expect the normal starter calculators to be useful.
Be very attentive to off-smells and aromas doing this. Any contaminant organisms will be propped up along with your yeast. If you can work under sterile conditions (autoclave, flame, etc) you should. If you're ever going to pick up an infection this will be the likely way.