Like Zak, I thin to order, but in the paint cup, thinners first then paint on top, swish it round with a brush then spray some onto paper towel just to get it moving and clear any neat thinners from the brush. you soon get to know the correct proportions. Surprising how little paint you use compared with brush painting.
I read somewhere that pre-thinned acrylic paint, left in the bottle will break down and become unusable, depends how long you leave it but it's weeks rather than days.
Finally, not all paints and thinners are the same so it's best to use the thinners recommended by the manufacturer till you find out otherwise.
If in doubt, just slosh it on like wot I does, it seems to work for me.
Haddock.
PS read my e-mail.