If it is feasible, test your hay so you know your NSC/WSC/starch from the bulk of the ration (grass is trickier but still testable). Weigh the daily hay/forage ration, then figure the total NSC/etc in pounds/ounces/whatever measurement is easiest for you (for whatever reason I prefer to work in ounces, but need to get more comfortable with g/kg). Then do the same for the feed. Good quality protein and quality fat sources are your friend here, if you can pack more calories into a smaller ration you have a touch of leeway with the NSC.
So NSC weight hay/forage + NSC weight feed = total NSC weight. Divide the total weight NSC by the total weight of ration, then multiply by 100 to get the total % NSC of the ration. Ideally that should be under 12% for a Cushing’s horse (10% for IR and laminitic cases).
With feed, look at the feeding rate as well as the NSC. A lower calorie 11% NSC feed that needs 8 lbs a day to get the weight gain you want (0.88 lbs NSC) versus a higher calorie 13% NSC feed that only needs 5 lbs (0.65 lbs NSC) to get the results you’re looking for is going to be more beneficial.
I’m pretty sure I have my calculations and numbers right, but it’s late and all of my references are at work, so I’ll check back tomorrow morning on my accuracy!