I think you just really have to decide who you want to "be" to your kids and try to stick to it. If you use the F word in your life, and around your friends (totally not judging if you do), then there is no use pretending to be a goody goody to your kids, beyond just trying your best, like you are. It will slip occasionally, so you handled it well.
I'm sort of not fond of that word, I do slip and use it occasionally with very old friends etc, but I generally do not swear at all in mixed company, that's sort of my own comfort zone. And I HATE it when frineds use it around the kids (and they do, non parents can't suddenly curb it, I've found :)So I haven't had the mishap of using that word with the kids. I think I have slipped and said "shi__" before...but they were too young and didn't notice :) Anyway, be natural and the version of "yourself" that is modeling control to the extent you believe is proper. Which it sounds like you are.
One disturbing thing about my dad (not really, but just saying...) he was EXTREMELY controlled about swearing my whole childhood (my mother never swore and still doesn't). If he was BEYOND FURIOUS, he may have uttered "dammit" or "hell" and my brother and I were SHOCKED. He didn't apologize, but it was so rare, we knew it was SERIOUS and we wouldn't dare do it :-0. Well, now that we're adults, he does swear around us, which is oddly uncomfortable for some reason :) SO sometimes I worry that if I set "too much" of a pure example to my kids, I'll scare them later :) So I'll try to keep it as close to my own standards as possible, not perfect, and not letting them do it of course! :)