There's no way she can maintain that weight on only 10-15# of hay a day. If you're feeding typical 3 string bales, each flake is going to weigh 6-7#. In a community feeding situation, she also natually gets more than her light horse counterparts because she has a bigger "shovel". The 5 horses would only be getting 5 flakes of hay twice a day. Weigh your hay. If she is only getting 15# a day, I'd get her thyroid level tested.
It's really hard to feed a horse that has special needs in a herd setting. Because she's low girl on the totem pole, you would need to pull her out and feed her separately. You also need to feed the fat daily for it to be effective. It's nothing more than extra calories if you feed it sporatically. It does make she coat look amazing.
What are you confused about? The math part? I'll try to not make it worse. In an EPSM diet, your goal is 20% of her total dietary intake to be fat. If she needs 30 MCals (30,000 calories), 20% would be 6 MCals (6,000 calories). Hay and grass had a small amount of fat (2-3%) so it's not really worth calulating. A senior feed that is 10% fat would have .1# of fat in 1# of feed. 1# of fat (2 cups) has 4 MCals or 1 cup would have 2 MCals. The 1# of feed will have .4 MCals (10% of the 4 MCals) in fat. Don't apologize for the hay. You have to feed what you have available. Alfalfa is actually fine even for an EPSM horse because it is typically lower in sugars than grass hays. It will be about 10% higher in calories than grass but it has higher values of most vitamins and minerals.
She's a very nice looking mare. Feminine and voluptuous which many Percheron mares lack. Have you tried researching her background? She's obviously purebred and with a docked tail whoever raised her had some draft background. If you know her foaling year, it wouldn't be too hard to look for her as she doesn't have typical markings. (black, small star)