I also use enamel thinner. But- they sell some real junk as generic paint thinner, and the stuff has lots of dissolved solids. So I hold out for "mineral spirits" or "turpentine". I used to use Testors thinner in the half-pint cans, but it is getting hard to find, so I use turpentine or spirits now. Not quite as good but close.
For thinning I usually start at 1:1. You do not need to be real accurate when thinning- slight changes in pressure, distance from surface and technique can adapt to different thinning ratios.
BTW, I use primarily Testors square bottles and Model Master, so the 1:1 ratio is primarily for those.