I'd get a dropper, a shot glass, and do a dropper full of glycerin, and a dropper full of rum, whiskey, or other strong drink.
Its really not advanced chemistry. After its mixed take a dropper and examine how thick it is. Too thick, add more liquid, to thin, add more glycerin. When you think you have it right, bottle it.

Every
e-liquid manufacture has a different idea of how think the juice should be.
I've also shaken bottles and examined how fast the micro bubbles rise to the surface. After a minute the bubbles should be almost nonexistent, only the finest micro bubbles remain. If you can count the bubbles, its too thick 8-o