you might look for a copy of PO Ackley's Wildcat cartridge loading book - then remember it was written a LONG time ago and that he often didn't mind pushing the upper limits. But it might give you some suggestions for powders and charge weights.
There are probably a half dozen or more different versions of the .444 Marlin case necked down to .358" and it's unlikely that any two of them have the same case capacity. Using data for the most common which is the .358 JDJ which also has the MOST case capacity of any of them I've seen is not a wise idea and can even be dangerous if your example has a significantly smaller capacity.
You really need to know which version you have who made it and get data from them. If you can't do that they you are flying blind. You are then working way out there in the dark and unless you have pressure testing equipment or pressure tested data from the barrel maker you are highly likely to be exceeding the safe pressure limits for the Contender G2 frame long before you realize it.
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