Dram equivalent back-calculating

Ok, here’s what I know: dram equivalent gives you an idea of the muzzle velocity of a shot charge. It compares smokeless powder to the performance of black powder. A 3 3/4  1 1/4 denotes a shot charge of one and one quarter ounces exiting the muzzle at 1,330 fps. It’s the old “high velocity field load”. Therefore a 3   1 1/8 is one and one eighth ounces of shot moving at approx 1,200 fps. A 2 3/4   1 1/8, then, is the same one and one eighth moving at approx 1,145 fps.

What I don’t know is how to back calculate a load, say 1 1/8 ounce load at 1,000 fps. Is that a 2 1/4  1 1/8 (approximately)? Anybody got dope on how to do this?

Be happy to answer if I ever, and I have no hope, understand the question..