Figuring out equity in your head, during a hand, w/out PokerStove (newbie question)

[rowhousepd]

So, there are 21 combos in his range, 8.5 of them beat you and you beat 12.5.
Your equity is 12.5/21 =~ 60%

So once you narrow down the hands in the villain's range, you think about the math in terms of combos?