The ammount of hash you get will depend on the thc content of the plant, for instance if you have 15% thc levels in that perticular strain then for every dried 100 grams of plant material you would get 15 grams of hash.
Don't think this is true.
First of all, THC testing is typically done one the best of the best material from a plant, ie hand-manicured top buds with no stem or leaf. A plant with "15%" THC isn't actually 15% THC by weight. . .just the manicured top buds are. That's OK, but most people aren't making their hash purely out of tip-top quality buds like that. Instead they're making it from leaf trim and/or low potency/low weight "popcorn" buds. Depending on how much leaf and stem is in there your "15%" THC plant may really only be 5-10% THC by weight.
Next, water hash is basically just broken-off and concentrated trichrome heads.
Because you're not breaking off every trichrome head, and some of the are fragmented and pass though your screens, you're simply not going to capture every possible trichrome in your hash. So even if your plant material had 15 grams of trichromes in it, you're not going end up with all 15 grams in your hash. Making hash from trichrome heads also doesn't capture any cannabinoids NOT in the heads (ie in the stalk of the trichrome or in the plant leaves) and that's also a potential source of some loss as compared to bud testing.
Lastly, each trichrome isn't pure THC either. The best of the best full melt has might be as high as 60% THC when fresh, but most hash is going to be significantly less.
A more realistic target is that you can expect to return about 10% of the weight of your trim as hash, which itself may be 20-60% THC. Exact numbers are going to depend on the quality of the materials you're starting with, as well as the efficiency of your extraction.