Always use the amount of butter on box of mix...
from my experiences, i always end up with too much oil and not enough pot. as long as you don't exceed the 7gram : 1 oz. oil, you'll be fine.
as long as the pot has enough surface area of oil to get heated up, you're fine. just add more weed. if you can, add KIF or Hash in lieu of pot. i cooked with about 5g KIF and got the best brownies ever...ever.