Thinking there is a multiplier for backs is just too simplified. As I mentioned, some front-back combos are near impossible. For example, old mill and Tolstoi are not particularly tough backs, but in some players, they can be downright impossible. Perhaps with commons, maybe PB, Sov 150/350, some old mill, Tolstoi, EPDG, and cycle 350 can have some multiplier (not sure what that will be)( and that is simplified, as there is nuance to each of these. But once you get beyond that, it’s all about the known population of the front/back combo.
Here is what I would do. First, familiarize yourself with the PSA and SGC population reports. Links below (in typical SGC fashion, they fucked up their population report and now it total eats shit (happily, Ryan Hotchkiss), and because you now need 8 characters for anything, you must type in t206 and then the back (cycle, hindu, etc))
https://www.psacard.com/pop/t206?byback=0
https://www.gosgc.com/pop-report
Next, I would sit down and read this Scot Reader’s book/paper on t206 (link below); it totally kicks ass.
https://oldcardboard.com/t/t206/Insi...al-edition.pdf
Last, I would search net54 for t206 threads and read as many as you can, with emphasis on Ted Z’s running thread