Tough question to answer easily
There are several factors which can determine the amount of preferment used in doughs. It's a difficult question to answer because there are so many variables. Including, for or instance, the type of preferment you are using (amount of liquid content, sugar, use of salt, etc). What may help you determine your preferment is to define your goals (time, flavor, texture) for the ideal end product.
Thebighat is correct in going with percentages, there is a good web page addressing this below.
As with most things, "less is more". Less with a longer rise time will increase the quality of the flavor and texture of the bread, assuming it is not over risen which will create off flavors and texture. It will also increase the shelf life of the end product.
In general, the more moist and elastic the dough is, the less preferment can be used because the liquid serves as a medium to increase the rate at which the yeast metobalizes.
'A preferment is used to develope the flavor of the bread by allowing your culture and yeasts to fully develope a portion of your dough so that when you mix your final you will notice a more developed and complex flavor. A preferment may also make it easier to shape breads like baguettes by making the dough more relaxed and easier to stretch without it wanting to retract. Different hydrations of you preferment will give you different characteristics in your final breads, and combinations will produce a result where two desired results might be achieved to produce the perfect loaf. It gets really complicated on the one hand, but it is never dull if you are willing to experiment with your culture.'
You may find the following links and sources informative:Factors Effecting Fermentation
Baker's Tip: How to Control Crust Color
For the right dough or batter temperature, find that friction factor
"Volatile Organic Acids In Pre-Ferments For Bread"Journal Of Food Science
Hunter, I. R.; Ng, H.; Pence, J. W. 1961.