I am by no means an expert on QL, heck I cant even figure out half the settings. I can tell you this, it has been very accurate for me thus far. I cannot tell you that such and such load has this pressure, but can show you where QLs predictions were within .5 grain or so of being the predicted velocity in my rifles.
Like any other loading reference you must start lower and use good judgment in devoloping your load. Where I have found QL to be extremely helpful is in its ability to find the right powder. The powder that goes with your bullet and COAL like peas and carrots. As you know some burn rates match certian bullet weights better than others in different length barrels.
I am guessing here because I am in no way a ballistician or expert on internal ballistics, but feel that the limiting factor we all face is the intitial pressure spike. If a powder is the appropriate burn rate it seems that the initial spike will be less and that pressures past this spike will continue to be higher through out the bullets path down the barrel. Based on this IMO there are instances where one powder can give you greater velocities than other powders with less pressure. I keep hearing velocity is pressure from some people and have to agree to some extent, but timing of the pressure curve seems to give a different outcome.
I have only used QL to work out loads for myself in about 15 different custom rifles
. In every instance I had my best results with the powder that QL predicted to be the best. In a few instances I learned that a lot of folks pet loads were not even close to being the powder. I may not use the absolute best powder listed by QL because of other factors such as temp sensitivity or availability but always try and use one of the powders that is predicted to be a top performer. It has worked out very well thus far and some of the powder choices are sort of surprising to me. So surprising and seemingly counter intuitive that I have even resorted to using a string to pull the trigger on a 6x284 that I couldnt get to shoot as well as I would have liked. I finally found an acceptable load using QL and the rifle shot pretty well, too bad I had toasted the throat using data from every other source before I bought quickload. Everytime thus far QL has been spot on.
It has also worked very well in working up loads for wildcats. For example my 22-6.5x47 Lapua. When working that load up I used another case with a similar water capacity (I think it was the 220 swift) made adjustments to the case capacity and pressure paramaters and ran the program. I backed off the QL prediction and started working up toward the predicted max. By watching the chronograph and pressure signs (even if it is like reading tea leaves) I worked up to about .5 grains over QLs max. My velocity and presumbably pressure mirrored QLs results.
Until strain gauges become readily available for the handloader I would say a chronograph and quickload results with a bit of good common sense and independant verification where available is about as good as you can get when working up a load.
But then again I do not know that much, but I am learning from all of the posts that more experianced guys here share.