What load are you considering? We need to know bullet weight and if it's lead/plated/jacketed, at a minimum.
800x is often regarded as one of the worst metering powders out there. However, that is about the only complaint you will ever hear about it (though it seems that was enough, as it is now discontinued). The powder is noted for its great versatility in hot handgun rounds, including the .40 S&W. According to some of the people I have "heard" type on the subject, who seem to know their stuff, Titegroup, 700x, W231, or any such faster powders are the wrong way to go with the .40, if you are trying to make cartridges that approach the intended ballistics. They definitely seem to prefer Blue Dot, Longshot, 800x for this purpose.
Last weekend I chronied incremental charges of four powders (Herco, SR4756, Longshot, AA#7) that were all very close in burning rate to 800x. With a ~165gr cast lead bullet I ran each powder from loads so soft they wouldn't work the pistol to as scary hot as I dared. With any of them a 7.2 grain load would have been very workable, maybe a little hot in the first two powders and a little light in the last.
I don't know why the published data gives you so little room to work, maybe IMR figured there wasn't enough demand for an obsolescent shotgun powder in a whiz bang new pistol cartridge to justify a full development program. I've never used 800s but frankly this is a place where I would ignore the data you have. I would start with a load likely to be very soft, and load one or two of various increments up to and maybe a little over that number you found. I would shoot every one of them over my chrony, and check every single case for signs of pressure, stopping the test if any appeared. Then I would note which charges gave me the velocityrange I wanted and I would start to experiment for accuracy within that range.
Of course, I would be relying on my experience to make the many decisions along the way. Since you state you don't have much reloading experience, this strategy might not be a good idea for you. You might need to let the 800x gather dust for a little while until you gain some experience with the procedure and the cartridge. I sympathize, its tough to be a beginner without any experience or published data to work with.
