Rules on live rock?

I need some assistance. I've seen different rules on live rock and live sand, mostly around how much should be in the system.

My questions:
What is the current rule of thumb on how much live rock/sand to have in a tank?
Is the per gallon number for the entire system or the individual tanks?
Could the number be for the entire system?
How much of that number should be rocks and how much sand?
I am fairly sure I have a *bit* (*=sarcasm font) extra. What should I do with it?
Should I let it die and store it for tank redesigns?
Should I offer it to the LFS?
Should I add another tank and let it live?

What do you think?