I agree with Gary, along with the idea, of not adding everything at once, let the tank cycle, and there may not be very much of one if your live rock is still in good condition, but there should be a small one. Once the cycle has passed, add your critters in small groups, a bit here, a bit there, nothing all at once remember, your maintaining a delicate balance and you ahve to give your tank time to adjust to every addition. Eventually, you will get to the point where you think "I don't need anymore clean up critters" and poof, you have arrived at your laymans method of calculating clean up crew density. (which is technically impossible, different snails and crabs eat different things, algaes, detrius, so on and so forth, some are large, some are small, in the end, you just arrive at what works. Also, some of these animals are not long lived, so you will simply need to refresh what you have from time to time.
"Maybe in order to understand mankind, we have to look at the word itself: "Mankind". Basically, it's made up of two separate words - "mank" and "ind". What do these words mean ? It's a mystery, and that's why so is mankind." ~ Jack Handey