Well I'll tell you how I figured it out with my horse (who as you know has been on the gaining streak for awhile.)
I took my horse's weight into consideration and compared it with the weight he was supposed to be at. I started giving him two flakes of hay but then increased it to three since he was a bigger horse he needed more hay to keep weight. To gain weight, and nutrients, I started to feed him 1 scoop of sweet feed as it had a good fat content and a little of everything. I then told the barn manager that I wanted him with a higher fat feed, so we added half a scoop of Triple Crown Senior (the barn manager recommended it.) After 2 weeks, we saw improvement but it was very small and since I was working him more, he still wasn't in the clear. So I talked with the barn manager again, and we doubled his rations (over time, mind you!) and that helped. But then recently I decided not to have him on sweet feed but keep him on senior as it doesn't have all the molasses and the senior feed had more vitamin a and minerals, so we took him off of the sweet feed and increased his senior feed again. So now he's at 2 scoops of senior, and 3 flakes of hay at every meal. He's gained over 100 pounds in 2 months, and has been able to muscle up without getting ribby.
It's about starting at a baseline that makes most sense. If you are working him lightly, and you are happy with his weight and he isn't a tank, start with 2 flakes of hay and maybe 1/4 scoop of grain. Then when he starts working, you can start increasing the grain ration until you are happy with where it is at. Probably no more than 1 and a half scoops when you're riding him more and he's burning calories and fat. But it's up to you! Make sure you have a conversation with the barn manager so you're on the same page.
Good luck and have fun!