"Seriously flawed"? Certainly not. It gives a good enough approximation within an acceptable margin of error considering that our opponents exact hand is unknown. Sure, it might be off by a few percent, but that difference is surely smaller than the uncertainty of the exact range we are up against. It gives us a good idea if we have enough equity to continue. If the numbers are far from close (we easily have enough equity or we clearly don't), our decision won't change based on the error. If things are really close, it is just going to be a marginal decision either way. We could be off on the number of outs we have/need by more than the percentage. In that case, the error of the calculation isn't our biggest confounding factor.
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