First of all....Pat yourself on the back for bringing her home!
Second. Be very careful about adding grain to the her diet. Free choice good quality grass hay and some alfalfa is best. If you really feel she has to have grain (and I know I would be feeling that way) then go get a bag of Senior grain. Its easier to digest. Im a Purina fan, but may not be available in your area. And a cup am and pm is not too much. Follow the 10 day rule. Let her adjust to new found feed and dont make changes, unless vet say otherwise, less than 10 days apart.
Gives her time to adjust.
And make sure she has access to salt block. So many hungry horses are not given salt at all. And good clean water.
We saved a herd of 9 using this method. The worst one of the group was a two year old filly that looked like a weanling. As we were loading them to bring them home she layed down and gave up. All we could do was pick her up, load her in trailer and hoped she would be alive when we arrived home. She was. And many years later she is healthy and producing foals.
When we brought this herd home we were very careful not to use to much of a good thing too fast. And I am happy to say. We didnt have a sick one as a result.
Good Luck...and again....GOOD FOR YOU!