I did some calculations of the change in vertical due to 0.1 grain of H322 in .223 and 6PPC. I think it was on the order of 0.03 inches for each. This will differ depending on where you are on the typical "sinewaves" of powder/velocity/vertical response (read about that other places).
However, on the UPPER end, with my 6PPC, I found a definite point at which I would get flattened primers and then a pierced primer. A couple tenths there migght make a difference. I pulled back 0.5 grains or more, because temperature also can play a role.
On the LOWER end, I have used charges several grains below what is listed, and I assure you, in a rifle, the bullet will still come out. When you're talking Mach3 for a high end load, unless you crimp the **** out of it, that bullet is going to move along smartly. 3.0 grains of Win231 reliably moves my 148 grain 38special lead bullet out of the barrrel at 500 fps. Moving a 60 grain .223 with many many more grains than this is definitely going to ignite.
I've toyed with my cheapie Lee powder thrower and found it was pretty good. The ones on my turret, which are also Lee, are also pretty good. I think they are generally +/- 0.1 but that is just "generally". I know that benchrest competitiors have far more expensive ones, but a lot of them just use their thrower and don't weigh (others seem to be switching to the automated dispensers for rifle, which is what I did -- and love it)