Saipro:

Congratulations on rolling up your sleeves and getting into the thick of things. That's my preferred route. It also simplifies maintenance and troubleshooting.

I'll attempt to help with my little knowledge base. In order of the questions,

1. You're safer going with ImP rather than IsC in your calculations.
For a place with overcast skies, your instantaneous best recorded (might bel lesser than the best seen) is great. Expect no more and you wouldn't be disappointed; actual NOC values are usually lower ... much lower (assume 73 - 78% of ratings).

Oversizing your array wouldn't be much of a hassle so long as you do not exceed the rated VoC of the FlexMax.
18 panels would be 4.5kWp of PV array. While it might not be the "cost effective" array size (depending on your battery bank size), if it makes economic sense to you, you're alright. Your FlexMax would hardly see beyond 3.5kW from that array size anyway.

2. I'm surprised there ever was a time your FlexMax and the Must inverter ever agreed (except on lower voltages. That, indeed, is an oddity. However, a 2V voltage drop suggests either significant loading of the inverter (based on battery bank sizing, again) or a drift in the calibration system of either (or rarely both) inverter or CC. I suspect you've learnt to place more loads on it even as you've discovered you at times have surplus power generation. A good voltmeter should inform you on which to trust (my money is on the FlexMax again).

3. Mine shows 100% on absorb voltages and 86 - 88% on float voltage (both the 24V & 48V Must inverters). Yours apparently recognises float as 100%. A wee bit preferable to mine but neither is accurate. Do not rely on it, ever. There's more to capacity estimation than mere voltage readings as it dynamically changes with loading percentage. The load % counter/meter is however pretty accurate across all models I've seen. It's the only thing you can really count on - up to around 35 - 50% loading. Right there (and beyond that), the inverter becomes more "efficient" (a paradox). At that point however, some older models might show waveform distortion.

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