First of all, you're going at this the correct way, via use of the chronograph. Last summer, using my Taurus .454 (6 in. version), I started at the Cast Performance level and increased in .5 gr. increments also. I shot 10 rounds each level, five through my Chrony and for accuracy and five for just accuracy. I also recorded the ES and SD for each load. As Marshall indicates in his Tech Manual (I'd sure recommend it's use), the smallest ES and SD may not be your most accurate load, but it most likely will be very close. I found the velocity increases to be fairly linear (approx. 40 fps) until the increase from 25.0 to 25.5grs, at which time it only increased 4 fps. Obviously a maximum load. The best I could get from the 6" Taurus was 1370 with the 360gr. cast bullet. I found the most accurate load at 24.5 gr. and the smallest ES/SD at 24.0 gr. All in all, was comfortable that I'd found the safe max. load for my gun and the ES/SD closely tracked the accuracy
One additional suggestion that may not be pertinent to you, but has been for me. I found that I had more accurate results when I repeated the same process on two separate days. I've consistently found that even when using the same loading set-ups, weather conditions, bench rest, etc., that the results do vary more than I expected. May only be sloppy work on my part, but if so, I've been consistently sloppy (a virtue?). Anyway, may be interesting for you to try sometime when a particularly strong masochistic urge hits.
A question, have you had the opportunity to test various primers and brass in addition to your powder testing? Different primers and brass, as well as trimming your brass, can certainly affect the accuracy. There, see, with all this help, you're going to so fatiqued.........that you wouldn't have any strength left for taking the garbage out!!!!!