Also note, if you use a PlusPower or something, it only adds to any existing final damage. It wouldn't be (20+10)xheads, it would be 20xheads+10. Also, if it does 0, PlusPower wouldn't add to that.
(interestingly enough, there was a funny situation I saw once where someone asked about a 'flip until tails' attack for 30x, got a tails instantly, and kept protesting it did 30 damage - even when two judges told him otherwise)