This is how I think of it......
the lower you set the psi the greater you have to reduce, but it means less overspray (good for the painting and your lungs)
there are times I've seen some people go 1:20 (paint:reducer) and 10psi when doing fine detail. it just means more passes to get the colour intensities you are after.
reduction ratios will vary depending on the brand /type of paint you are using, needle size being used, current air temp and humidity you are experiencing.
unfortunately no magic recipe.