Throwing powder at the range is easy if you live with the parameters you set.
1. You'll want a measure which is easy to convert some kind of value "on" the measure to grains. Ie, my Harrell Premium "meter" uses clicks. Example, 113.5 on my Harrells relates to 113 on the drum and an extra 5 clicks. It will throw 69.9 gr of Magum propellant. Their are 6 clicks inbetween each number. The custom 90 has 4 if memory serves me. Here is the BR premium link just to show the culver drum with graduations. Some of the Harrell models will throw to 120 grains; big enough for your AM?
New BR PREMIUM Powder Measure
My old RCBS Uniflow with the micrometer insert will do nearly the same thing but without the high cost and the clicks.
2. You'll want a powder which will "meter" well. I feel every powder has it's own merits. I've acutally tested some long grained propellants which metered decently well. Some don't!
3. You'll want to preestablish a Hi/Lo range for your charges. An example would be +/- .1 gr, .2gr, .3gr, etc. Obviously for such a cartridge as an AM your percentage can be larger.
4. Before heading out, run a range of throws to predetermine where you'll want to start and where you'll want to stop when you've hit your predetermied max. Example, when I ran a ladder test last month for my 30-06Ackley and the 200 Accubond, I had determined that 68.9 gr of Rel 25 was max via QuickLoad. I subracted 10% and had my bottom end. Actually, I determined 21 shots at .3 gr variance I came up with 62.9 gr. I then found out what click value gave me an average of 62.9 gr and that was my low at the range. I then determined what click value gave me an average of 68.9 and that was my high. I've stopped before the high when pressure signs showed max and sometimes even continued beyond when pressure said to keep going.
Hope this helps.