How much yeast to pitch?

Okay, I’ve brewed a few extract batches and most have turned out great, but not I feel it’s time to get a bit more serious. For all my kits, I’ve been using Wyeast and already have the yeast for the next five-six batches.

Question - haow do I know how many slap packs to use? For anything under 1.060 OG, I’ve used one and anything over I’ve been pitching three.

Without getting into starters yet, am I on the right track? Over or under doing it? What’s the actual math I should be using?

Thanks and Happy New Year!

Here’s a real handy pitching rate calculator:

http://www.mrmalty.com/calc/calc.html

Thanks Glug! That’s just what I was looking for!