There's no hard and fast rule for how much to thin paint — even if you read (for example) to "thin Tamiya acrylics 1:1 with X-20A," what about older paint from which some of the thinner has evaporated, leaving it thicker than from a brand-new bottle? And that's without even considering that different brands and/or types of paint will probably require different thinning ratios, let alone the question of what you use to thin the paint with, because that will also matter.

My normal method is just to add in some thinner until I think it looks to be enough, then try spraying. If it's too thick, I add a little bit more thinner, and if it's too thin, I put in a little more paint  IMHO your best use for those eyedroppers is to put some thinner into them and just squeeze out some into the paint until it sprays well.