I use 9.5grs of Unique with a Lee 200gr. RFN boolit in my 20 inch Marlin. It clocks 1350 10 feet from the muzzle. Groups at 50 yards run in the one and half inch range for 5 shots with peep sights.
If you want to see how accurate your Marlin is you need a good scope. If that is not a option then a Williams receiver sight and a Lyman 17-A up front at the very least.
My rifle likes .432 or larger boolits.My mold for the Lee 200round flat nose drops at .434X.432 and I size .433 in a H and I lube die. Other molds I beagle................Terry