Congratulations on getting to the next level gc.
For your area Penn State, Rutgers, or VA (do they have a turf dept?) would probably offer the most relavant advice. Purdue is certainly fine. I am partial to Michigan State
Best thing you can do for yourself and customers initially is develop a keen awareness of the predominant soil types and grass species in your area. Then begin learning what those grasses require annually and how those soil types handle fertilizers.
You are going to arrive at a range of needs per thousand square feet (M, some use K as abbreviation) per year. That baseball field might be a good place to experiment and they might like to have a couple free or cheap fertilizer trials. Anyway, your reading and talking to others will help you arrive at something like this for your area:
Low maintenance and overall standards 1.5# nitrogen/M per year. Probably does not treat for weeds, mows every 10-14 days, non-irrigated. Many places that are truly low maint don't fertilize at all.
Medium Maintenance maybe something like 2.5# N/yr
High end could be something like 4-5# N/yr (I do not have any athletic fields that use more than 4#/yr fwiw).
Then it becomes how should you deliver it (apply), how often, and when-a slightly different question from how often. Most cool season turf is better off with a big chunk of its total nitrogen delivered in Fall. It ell places that only want to fertilize once to do it late September to early October (southern MI).
Distinguish yourself by buying and using good products. My customers appreciate that I use high end slow release products and do not have to come as often. In Fall I will use all mineral (cheaper) fertilizers or low percentage slow release. You can learn a lot from your dealer/distributor. I don't buy fertilizer from them any more but John Deere Landscapes would be a good place to start.
I am not ignoring P and K, just trying to get you started on N, which is the key element of turf fertilizer plans.
Also, read about the nitrogen cycle to learn how various sources of nitrogen break down and become available to the plant. That should get you started.