What the manuals don't take into consideration is the fact that most brands of brass have different internal capacity. The likes of Lapua has more capacity than say, Hornady, so a specified load in a manual might be 3,000psi down when used in a Lapua case. I find QL invaluable, but as stated, you can only confirm accuracy on the range of in the field. There is no substitute for trigger time.
Jonathan.