Is this guy trollin'? I sincerely hope not!
I don't have any engineering experience - but I think between what you've already found (online calculators and such) - and maybe the information presented in this book (which has a section or two devoted to the mathematics needed, and how to work the problem, to answer your questions) - you should be able to get the answer you need.
BTW - if you're wedded to the dual-chain system, then so be it. I just wanted to point out another resource to you, in case you weren't aware of it (and for others, too), because building a working tread system for larger tank-style robots is a challenge. Nothing inexpensive exists on the market; what does exist will figuratively set your wallet on fire.
Good luck with your project, and I hope you find your answer.