OK Popper, I finally got a couple minutes free time to run some numbers for you. I like to see port pressure at least 6,500 PSI for cycling a 300 BLK with heavy subs, up to 10K is fine. but that's with a max .125" hole run wide-open. 2400 should cycle a pistol system fine with your 145-grain bullet anywhere from 9.0 grains on up, might start at 10.0. 10.0 grains with the bullet base at the bottom of the neck should be good for about 1,300 fps and 20K psi peak pressure with 9K psi at a the port and decent dwell (4,545K muzzle pressure).
RX7 should take about 14 grains to start cycling and give 1,300 fps.
With 4227, start at around 12 grains.
1680 should take about 14-15 grains to get the same port pressures and velocity as the others (meaning around 1300 fps), but I have had zero luck with 1680 at anything less than full-power loads in the BLK. At low pressure it's unbelievably filthy and very erratic/inaccurate. The unburned junk is like ground black pepper and gets everywhere in the action and magazine.