When I use the 500 grain bullets I use 25 grains of 2400 under them. This fills the estimated remaining open space of the case about halfway. So, in theory, if I tip the case sideways, the powder level should be right across the flash hole area. If it dips forward slightly, the flash from the flash hole can dangerously detonate the entire powder charge at once instead of burning it from back to front as it should. This is why I use a bit of polyfiber in these loads. I weigh the wads and they weigh roughly 1 to 2 grains each. Basically they are enough to just hold the powder back. I'm definitely NOT stuffing the cartridge at all...it's a very light amount. I've noted that my accuracy is better (believe it or not) when I use the wads with the same given load above.
By the way, as a warning, this load is only used in a STRONG and MODERN rifle....a Wesson and Harrington Buffalo Classic. I would not even use this load in my Thompson Contender.