Most manufacturer data sucks, and at least for pistol rounds is often very conservative due to liability. What is hilarious is if you find old manuals with powders that
are still made today, some of the old manuals have much higher max loads. This is probably because "lawyers got involved". The most "ambitious" manual I've ever seen was the Speer manual. That one, there's actually hot shit in there, to a degree. Like at one point when I used to do ladders, in the other manuals I would pretty much start midway up from the start load. The speer manual on the other hand I would legit start at the start load because that's how big of a difference it usually was. Some of the loads I ended up using were still below the speer book max for relatively the same combo listed in the manual.
Whenever I used VVs data for pistol rounds I basically would start close to the max load, and contrast those values with "stuff I found on the internet" to find out
where the real max was, etc.
Honestly as far as testing goes you really have to do all your testing yourself, with your own "bible" as I call it, your own chrono/range, and your own guns. There's too
many variables in play. A lot of the manuals use test jig velocity readings which are almost worthless in the real world. (I think with Speer at least on some accounts they
tell you the gun they used for testing, at least. ) Temperature with some powders is a huge variable, too.