Hello... I think it was you who e-mailed me but here is some information to share...
How much do your "flakes" weigh? First of all, get a scale (I bought a 22 pound scale from WalMart online for $25) and make sure you are consistent with their food. If needed, make sure each one eats separately. It's impossible to have a horse on a specific diet when they share... one may get more or less than it needs, particularly if you are cutting back to a strict diet.
At any rate, here I measure a "flake" to be seven pounds... our bales are 110 pound three-wire bales.
Second, get a weight tape. They are available for under $5. Weigh each horse and feed them 1% of their weight per day, but DO NOT go under that percentage.
Third, what type of hay are you feeding? I would recommend to NOT feed alfalfa to easy keepers. Instead, feed a good quality grass hay (Orchard or Timothy is probably the best nutrition wise). Bermuda grass is okay but too low in nutrients, if you want to feed up to half of their hay ration as Bermuda that might be okay. But they really need nutrients.
Fourth, don't give them ANY grain. It won't do them any good. You can give them a little bit of soaked hay pellets or beet pulp soaked, and add a good multivitamin dosed per manufacterer. (Example, take half a cup of beet pulp per horse, and soak it. Then give them Grand Vite supplement... bucket says one scoop per 1000 pound horse, so add 1/4 of a scoop to the soaked beet pulp.) The grain is probably made to provide vitamins, but if you don't feed the recommended dose (which may be too much of the grain) then it's basically no good. When I give grain for vitamin content, I just use a vitamin supplement (I like Grand Vite but almost any will do). They don't need the carbs from the grain, even if its low carb.
Fifth, really DO try to exercise them more. Diet won't do any good without a good workout. Yep, it takes time and work but that's what you signed up for when you got horses.
I hope any of this helps. I personally wish I had a personal trainer... my own horses are always weighed, have their food weighed and monitored... but the more you work them the more food they can have. My horses easily get 2% of their body weight and sometimes even a little more than that... and a lot of grain and beet pulp too! But, they are worked almost every single day until they are hot and/or sweaty.
If exercising, the key is to start out slow and don't overdo it.
Good luck
Andrea