I didn't calculate -- I come up with that number from real world use. Solar panels rarely if ever achieve rated output to the batteries under non-lab conditions.
My boat currently has a 225AH house bank and 200 rated watts of PV panels, and my little Danfoss-powered fridge uses about 3.2 amps when cycled on. My fridge is not very well insulated. When we leave the boat with only the fridge running it can keep up equilibrium when the weather is sunny or partly cloudy, but during extended multi-day overcast periods it is unable to keep the house bank to full charge -- it will lose a little daily over time.
My primary experience is at approx. 40-degrees north. Craig C (the OP) is further north than I am. There are wiring losses. There are shadows from rigging, sails, etc. Even at low latitudes, the sun is not high most of the 24-hours per day. There are overcast days, sometimes for a week or more. There are other power draws (e.g. AP, CP and instruments, lights, accessories, computers/tablets/phones and personal accessories).
When you consider all the above, that's where my 400w number comes from. It's an imprecise hip-shot and that's the way the real world is. My present setup is pictured below.