Weights for MX

I need a third weight for my MX as I'm piggybacking an FSQ106 on top of a TEC 140 and 2x 20 pound weights doesn't quite balance it. 

1. What is the diameter of the 10 pound weight vs the 20 pound weight? 

2. I believe I need only 6 additional pounds, would the 10 pound weight be more ideal or does it really make much of a difference (thinking about other scopes piggybacked in the future)? 

Thanks!

Rick