As John Taffin and others have pointed out, Marlins with Micro-Groove rifling can shoot cast projectiles very well. It just takes some systematic and careful load development to make it happen.
A gas check is a "must" if you're pushing things above 1400 f/s or so, as is finding a projectile with the right amount of bearing surface, the right alloy composition and a suitable propellant.
Mr. Taffin has been able to develop very accurate cast bullet loads for Marlin carbines in a wide variety of chamberings and has written extensively on the subject. Reading up on this work would definitely save you a lot ot time and effort coming up with your own formula.
FWIW, I shot a lot of small game and varmints with my old Marlin 336 in .35 Rem. using the same Lyman 358429 cast bullets I made for my revolver back when it was the only CF rifle I owned. Didn't own a chronograph back then, as the few available cost about as much as a good used car and used special media to start and stop the clock, but estimate that they were running about 1000-1100 f/s. Accuracy ran about 1/2" at 50 yds for 5-shot groups using a receiver sight from a solid bench and bags, IIRC.