Am I hurting my horse?

[SunsetOvrC@aol.com]

I weigh about 200 pounds and my Arab mare is only 14.2.  She has an average length of back.  Her loin muscles and general condition have improved considerably since I got her.  (she was underfed when I "rescued her".)  You could see the bones of her croup and the little hollow behind it, which has since filled in.  I have been trying to stay on a diet for myself and for her.  I know a lot has been learned about weight ratio to performance.  Does anyone have that formula that they figured out at the Tevis?  The one that showed that above a certain ratio horses failed to complete.
Not that I'm planning to hit the Tevis any time soon, but it would be helpful