Does anyone know how many mg of THC are in your average edible?

I've been making them for a while and would really like to find out. (google is no help)

Basically my problem is I have no idea how much THC is extracted into my lipid when I make butter.

My calculations go like this:

1/2 ounce of good weed (10%+THC content) used to make a batch with 10 doses

so 1/2 ounce = ~14 grams

Therefore, a 1/2 would have ~1.4 grams of THC.

If I assume that close to 100% of THC is extracted into the butter I make, then the batch of brownies would have a little less than 1.4 grams of THC. And each dose would have ~140 mg of THC.

But I doubt that 100% THC extraction can be achieved. Does any expert know what is more realistic? 25% , 50% , 75%?