The old standard has always been that your horse should be eating at least 1-3% of his body weight in food every day. I use 2-4% from personal experience because I believe strongly that horses without enough bulk to their diets are more prone to colic, ulcers, and behavioral issues like stable vices. Horses are physically and mentally intended to eat for most of the day. The more overall material we can give them, the better.
While sometimes 1% is necessary for founder prone easy keepers, I would rather try to cut calories and starches by making swaps in what they were eating than to cut the overall amount first. Seriously, have you ever tried to put a 1000 pound horse on a 10 pound a day feed plan? Trust me, it isn't pretty. You end up with a horse that is convinced they are starving and will likely start stall kicking or other undesireable behavior, which is likely to give you a bad case of rattled nerves. So, unless you have made all the swaps you can, and you absolutely must restrict intake further, I would stick with 2% or more.
I should say, that before calculating your 2%, you will need to remember that we need 2% of the horse's ideal weight, not their actual weight if there is a difference. This means that you will need to add or subtract approximately 50 lbs from your horse's actual weight for every BCS score other than 5. This is why it is necessary to know both those numbers. So if your horse's actual weight is 1250 pounds and his BCS is a 7, you will want to do your 2% calculation based on 1150 pounds (1150 = 1250 Actual Weight - 50 (BCS of 7 to BCS of 6) - 50 (BCS of 6 to BCS of 5)) When you calculate 2% a small 800 pound horse would need at least 16 pounds of total feed daily while a larger 1300 pound warmblood would need 26 pounds of total feed daily.
Guideline 3: Never more than 40% concentrates.
Research has shown time and time again that horses whose rations are 40% or more concentrates (grain, mixed and/or pelleted feeds) are much more likely to suffer from colic, ulcers, and behavioral issues. So after you do your 2% total feed calculation, take 40% of that, and make sure your concentrates never exceed that number. You will keep your horse physically and mentally healthier, and avoid wasting your money.