I have different Hays from different suppliers and, surprise, they all differ in weight per flake
So weighing some flakes gives you a pretty good idea. With grass hay I personally would go free choice, but with a slowfeeder haynet, so horse eats like it's meant to eat, a little, but constantly. Then, the amount of sweetfeed/grain/pellets depends on the workload. The feed you mentioned asks for a minimum of 0.3lbs per 100lbs of bodyweight for maintenance. Now, maintenance includes light riding for an hour daily. The minimum amount is set to make sure horse gets all needed vitamins and minerals.
So I would, unless horsie is already overweight, start with about the middle of minimum and maximum recommended ration. On top of free choice hay.