Unfortunately there are no hard and fast rules for hexapod robot payload. It’s difficult to quantify for many reasons. What is the acceptable current draw under the increased load? What amount of performance degradation is allowed before it’s “too much”. I have never added much weight to the CH3-R with 475 servos. I have tested it carrying a can of coke with no problems. I usually say around 16 ounces would be no problem. Using 645 servos allows a lot more weight to be added.