question

I reckon you cant calculate it and I assume we are talking boats.

Installed kW in starter motor has close to nothing to do with actual amps drawn while cranking. Temperature, amount and load of attached hydraulic pumps or gearboxes have a huge impact for example. Starting strategy of the engine too. Some silly things are going on in order to save the environment/monies.

The Victron AGMs have their CCA on the low side compared to starter batteries. Doesnt neccessarily have anything to say as the rating is close to useless for the real world (read up what is actually tested) , but might mean the voltage might drop far. We had issues before where cranking rpm was out of range or ancilliary computers that were required for engine start, browned out and everything after the second start attempt would have not been a successful start although she was cranking fine and the engine was still supplied with enough electricity. Your boat(?) sounds on the big side and might have a similar setup, computers talking to computers to do something.

As I know it, the engine supplier just states he wants to see, say, 180 rpm during start. The rest is then your problem. Battery size, wiring, auxilliary load. There is application people that will confirm that on paper for money coming from the supplier for you after logging a start attempt on the computer. Others may specify a suitable battery in the manual, then I would refer to that.

Do the batteries supply the "household", meaning are they actually cycled or are they glorified starter batteries? The first is bold as they need to start an engine when empty which they are not actually designed to do in the first place, the second is a lot of money for the wrong tool I d say.

Charging the battery then, 6h from cranking or from fully depleted? I would go with no as not chemically possible for the latter. Full means less than 1% of the capacity in Amps going in there and that takes longer in the lead batteries world. That bit of cranking yes, takes a couple of minutes to recharge the energy and under 6 h to possibly get to 1%. In this case lots of money spend for oversized kit. They shoot right through to absorption and 15A for the big batteries will be plenty to keep them topped up.

Overall are AGMs in series somewhat troublesome, especially if the are treated like you intend to. Excessive starting current, warm near the engine, charging should not exceed 10% of the Capacity in Amps anyways, you are close or over 30%. They then drift off, one cooking, one undercharging, both suffer and quit long before their time.