What percentage of bodyweight you feed depends on several factors, such as do you want to maintain weight, lose weight, gain weight? what hard feeds does horse get etc.
2.5% of body weight is total amount of feed per day for average horse
This can be reduced for overweight horse or increased for under weight
Feeding ad lib hay can prevent horse becoming stressed etc but some horses can be very wasteful and trample it into bedding.
Proportions of hay to hard feed depend on work done, as above whether you need to put weight on etc
Overfeeding hard feed when horses on little work can cause loads of problems, better to feed hay plus other sources of forage like Alfa A, Hi Fibre cubes, Speedibeet or Hi Fi Lite if over weight