I don't think I'd rely overmuch on QuickLoad. As brilliant as the program and its creator are, it is still software. That means that it will spit out pretty much what the assumptions and formulae you feed in create. Actual pressure traces are more trustworthy because they report what is actually happening.
BTW, I disagree with what 2525 says. If all the powder that is going to burn is ignited more or less simultaneously, then it will burn out more or less simultaneously. That occurs at the pressure peak; again, more or less.
Some powder never does burn. In a bottleneck case, it is probable that a "plug" of powder gets compressed and rammed into the back of the bullet at primer firing. That dense plug is too compact for primer gasses to penetrate and cause ignition. Only the rearmost surface layer can and will burn. Any residual burning from such powder kernels might explain why the post-peak curve is shallower than we might expect. Note, however, that with the pressure dropping, any post-peak burning is much less efficient than otherwise simply because the pressure is below the optimum for that powder. That's why we can say that the peak marks the end of effective burning.
In any event, by timing the pressure curve, it is quite simple to know where the bullet was at any given point on the curve.
