Multi-tabling help

[Goodnews]

How do I determine whether adding another table is worthwhile?

I know you hould stop if your hourly winrate stays the same, but wouldn't that take 50k hands at x tables for me to get an idea? If this is true, then isn't it possible for me to lose alot of potential money because I am playing one table too many for 50k hands?

I thought about taking my winrate 4 tabling, and comparing it to my rate 5 tabling, but this is probably the most systematic way of doing it. I was wondering is anyone else knew a different method?