potency will depend of quantity, and quality of the plant material used, as well as method, and of course the quantity of butter used.
I have found that throwing it all together in a crock pot over night produces very strong, very disgusting butter. (which most casual stoners cant handle)
for a milder, much more enjoyably tasting butter, add plant material to boiling water/butter, and simmer for 2 hours, cool, refrigerate, and poor water off the next day.
actually finding the thc content? im not sure.
hope this was slightly helpful.