The 650b ISO 584 mm (292 mm radius) and the 26er ISO is 559 mm (279.5 mm radius), so you have a difference of 25 mm--or 1 inch--diameter (12.5 mm radius). Therefore, the equivalent 650b tire on a 26er fork/rear should take up 12.5 mm more of the gap (+/- 1 or 2 mm for tire variances).
This is theoretical AND probably more inaccurate when comparing different makes/models of tires (e.g. my 26x2.4 Big Betty has a larger diameter than my 26x2.5 Maxxis DHF).
I guess the real question is: What's the minimum you gap you need for your riding conditions?
FWIW: I could see trying some skinny 650b tires on an Endo if I were looking at a XC racing; because you would lose tire profile while keeping everything else roughly the same. BUT I don't race, I don't ride XC, and don't own and Endo...
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Thread: 650b on a Chilcotin - with Pics!