I think one reason why my analysis, as well as yours, is very robust is because SFCs of ~0.7 are consistent for the types of Turbofan engines that US and Russia have access to. So while it's useful to vary that for robustness purposes, SFCs >.8 are probably not a realistic assumption. 2/x
In the case of the 9M729, the fact that it's probably some 0.5m or more longer, providing significantly more fuel space, further increases robustness. Again, my analysis is not about predictnig the exact range. But it's clear that Russia had some explaining to do, which it couldn't. 3/3
Forgot to add: In my case I also was quite generous with "cutting away" fuel tank for the foldable wings. In reality, and with appropriate silicon tanks, it's probably much less space that is lost.