I haven't used Aeromaster, however from their web site at:
http://www.aircraftcolors.com/
they say to use 15%-25% water for thinning (distilled water works best since it doesn't have the chemicals and impurities found in regular tap water). I think this is their web site, but I wouldn't swear to it.
You might just try some different things on some scrap and see what happens. 91% isopropyl alky is a good choice to play with. 50% alky and 50% distilled water is another good choice (MikeV recommended that and I tried it and liked it!; thanks Mike!).
I don't think I'd use Tamiya thinner. It might work great, but I just have this thing about mixing brands. I think their thinner is mostly alcohol (the contents say something like "Contains alcohol elements" but I can't remember the exact wording).