Ok, so don't need to gain or loose weight--just maintain. Plenty of forage---really good. As for the feed, you need to look at the bag to determine what is an appropriate amount to start with because all feeds aren't made the same. The bag will give you a general recommendation based upon size, age and work done and start there. Once you've fed that for a couple of weeks then you can assess by their weight whether you are feeding too much, too little or not enough.
I would be more specific, but I can't without knowing exactly which feed you are using.
For good information on feeding, go to The Horse Magazine's website and check out their nutrition articles. They are a great resource for horse owners.
Licensed Veterinary Technician