Roy, that's the chart I use today, I have it taped inside the pantry!
So I messaged the Flea Bay seller and it turns out that the number displayed is voltage, NOT state of charge. It threw me off that the listing indicates that this is for "12v lead acid batteries only" but that they are showing 50.5v in the illustration. Oh well, Chinese marketing....so it's basically a voltmeter with a cool looking SOC bar graphic added. Basically took my paper chart and baked it into the voltmeter with that gauge.
I may spring the $10 on it, if for nothing else so that DW has a more visual reference on how the batteries are doing before she turns on another light....