I weigh all my concentrate portion of the horse's diet. I have a digital scale (bought on eBay for my internet business for weighing for shipping, it works great for weighing feed); I picked a scoop to use to measure out feed (mine is a store bought frosting container); then used it to weigh out different amounts of the feeds I'm using. So my scoop is marked with 4oz line, 8 oz line and then full is 12 oz for my feed (I feed Progressive grass balancer). When I'm using different feeds, I have a separate scoop for each feed, so that I can mark on the container lines for, at least, 4 oz and 8 oz of the particular feed.
I probably should weigh my hay, but I like to give them pretty much free choice, so don't actually weigh it.
My two yearling fillies, both to mature B size are getting the following daily:
1. Progressive Nutrition ProAdvantage grass formula - 1.25#
2. 1/2 cup whole oats (3 oz) - more of a treat than actual addition to their nutrional needs (they were free)
3. 1/4 cup BOSS (1 oz)
4. grass hay pellets - 1#
5. mix forage pellets - 1/2# (just trying these out - before adding this, they were getting 1.5# of the grass pellet)
6. grass hay - 2-3 flakes (about 4-5 pounds)
[Their total daily intake is divided into two meals. The majority of their diet is the Progressive ration balancer and grass hay. I bought the grass hay pellets at a time when I could not get good baled hay.]
Edited to add: How much you feed will depend on what you feed and the manufacturer's recommendations for that feed. Also, my horses have white salt and clean water available all the time.