You don't want it going over 200 F. Do a water bath, and warm the cooking oil to around that temp, add the hash, and let it simmer for a while.
How ever much it takes to get you stones is about how much you should addX the amount od doses you want.
In order to make sure the THC is orally active, it has to be heated to a good 200 F.
Example (Numbers not accurate)
If you smoke 0.25g to get stoned, then add 0.25g/dose. If you want to make a 12 peice cake, the 12x0.25=3grams.