Every vendor should be able to supply you a manufacturer P-sheet (product information sheet) on the paint you're using which will include reduction ratios, reducer temp usage (if you're spraying at this temp, use this reducer), hardener info, base guideline pressure to spray at, etc.
For instance, I spray a lot of PPG. Deltron DBC basecoat is a 1:1 mix with the 2002 clear over it is a 4:1:1 mix. While DBU basecoat is a 1.5:1 mix and Concept clear with a speed hardner is a 4:2:1 mix. So you need to be able to know this before going in on the product and that's where the P-sheet for your product is a must. Not just for the paint and primers, but ask for a P-sheet on the reducers and the hardeners, too, as this will give you an idea of which reducer will flash out the best for a given temp. and which hardener will give you the best flow time for your paint environment.