That's exactly what I did. The max load was 4.4gr and I started at 3.9 gr.
Actually, what he said, and what is suggested, is that you should start 10% below the suggested
starting load, not the suggested MAX load.
Basically--if you are putting together a recipe that bears little relation to what is actually given in the reloading manual (and truthfully, you should always look at several reloading manuals, not just one), particularly if you are using magnum primers, you should start
extremely conservatively.
If you find it isn't enough to work the action, no problem. You just add more powder. You DON'T want to have the opposite end of the spectrum problem, however.
I note also that when comparing loads, you don't want to change the OAL if you are doing powder checks. Once you have a powder amount in the range you like, then try different OALs. (Carefully!) After some work with that,
then try combinations in your preferred range. However, comparing one load at one OAL to a different load at a different OAL is useful only if you want to compare those two loads. If you are actually trying to work up an
optimum load, that just isn't going to be the most useful way to do it.
Plenty of people use magnum primers in 9mm loads, and don't worry about damaging the flash hole. (I do on occasion, if I'm out of small pistol primers.) However, it
does make the round act differently, so unless you already KNOW that the difference is not dangerous and that the difference in performance doesn't matter to you, you should drop the powder charge a bit and actually go test it carefully.
When I'm working up a new load, I tend to make 20 rounds on the low end, and then 20 rounds for the next 0.1 grains of powder up from there for about 0.5 grains. I take them out and test them for slide action, recoil, a little bit of accuracy (not much, this is just an initial test), and across a chrono for speed.
That gives me enough info to see 1) what the load does in general, and 2) how much the load changes per 0.1 grain. (I'll note that you can't take that for granted, though--certain powders change smoothly up to a point, and then suddenly change drastically.)
When I've got a couple of variations that I like (a good solid workable load, and loads 0.1 grains on either side of it) I make 50-100 of each and take them out to the range for serious testing for accuracy, speed, and recoil characteristics. (I rarely change OALs, simply because I know what OALs feed reliably in my guns, so I stick with that. One less variable for me to care about.)
Also: powders simply do not behave like fluids. There are certain specific circumstances in which powders CAN behave similarly to fluids (notably, when under constant vibration) ----but a reloading press is not one of them. Indeed, empirical data is the way to go.
I note that this entire thread really needs to be in the reloading forum, not the training forum.