Smoking converts about 70% THC - the rest is wasted/burned.

Eating it doesn't do this, not sure about the efficiency but I would guess it to be higher so long as the THC is in a good state to be metabolised and you don't do anything lame like throw up

I would try making 1 brownie/cookie with the same amount that you smoke in a good session - then you will know.

I alway did this with Hash though - so no real need to make butter