In my experience, I've used a bit more oil than called for in the recipe and its been plenty to extract enough THC to get people with the highest tolerance stoney baloney with one brownie.

For instance, I use a brownie recipe (that fills a 13x9 pan) that calls for 2/3 cup veg oil. I usually use trimmings along with any little small nugs/shake I have (usually an ounce or more per batch). I then use a little less than a cup of oil, grind down all plant matter finely, slowly add to warming oil, then simmer (not boil vigorously) for 2-3 hours-this is a judgement call for you since you're using hash, stems, and duff. I would error on the side of simmering a bit longer. I usually do this and it has never resulted in poor oil-always potent.

Then after cooling down a bit and straining with cheese cloth or the equivalent, I'm left with about 2/3 cup, maybe a bit more (you add a little more than called for or use it to cook something with later-if its just a little over 2/3c, I'll just add it to the brownie mix, doesn't affect the cosistency).

One reason I have to add more than 2/3c is b/c its usually not quite enough to soak all the ganja I put into the pot at first. After simmering for a while, it all settles down/shrivels a bit leaving more. Plus, you lose a little that you can't squeeze out of the herb.

hope this helps, I know its not super specific + I haven't extracted from hash, so I'm not sure of the amount you would want to use per batch. Maybe someone else could chime in on that one...