I've always used Kleen-Strip Mineral Spirits which one can buy for about 2 bucks per quart at the local hardware store, and have never had any issues. Recently, I bought a bottle of MM airbrush thinner just to see if there was any poosible advantage to using it, and I found no difference between it and the generic stuff.
For additional cost savings, you can reuse your thinner for cleaning. I have a couple of spaghetti sauce jars that I keep my used thinner in. I put dirty thinner into one jar. After a few days, the paint particulates will settle out into a sort of sediment on the bottom of the jar leaving clear thinner above. After I've collected enough thinner, half a jar or so, I carefully pour off the clear thinner into my other jar which then becomes my source jar for cleaning thinner. The thinner does take on a sight greenish hue, but cleans brushes and airbrushes fine. By recycling that way, I can literally get years of use from a quart of thinner.
From my experience, I don't think there is a useful thinning ratio that can be applied to Model Master enamel because there is too much batch to batch variation, even for bottles of the identical color. I have occaisionally had bottles that sprayed perfectly straight from the bottle. I have had other bottles in which the paint was so thick that I had to add a lot of thinner (maybe four or five parts thinner to one par paint). I think you have to treat each paint individually and add thinner as needed until it's properly reduced for painting.