I am hoping that using float is not the culprit for quicker discharges after dusk over the last year. Probably not, might just be our high usage and more than normal heavy cloudy skies these past years.
Float, in and of itself, will not cause higher discharge, or capacity loss. However, its commonly recommended not to hold a lithium battery at full charge for extended periods. The question is - what constitutes an extended period? SimpliPhi specifies a 54V Float Voltage, and warranties their batteries for that value, so I assume Float from the time the battery is fully charged, to sunset, is not considered extended.
(For myself, I prefer not holding the battery at 100% SoC for any period of time, no matter how brief, so use a lower Float voltage that draws off a small amount of charge over the Float cycle, ending at 98-99% SoC.)
The FNDC and Optics are good tools for tracking battery discharge. With the FNDC, the SoC tracking is a convenient metric, but the critical measure is battery discharge in Amp Hours. Assuming a 400AH battery, fully charged (100% SoC) the recommended maximum discharge is 80% of capacity (80% Depth of Discharge). That would work out to 400 x 80% = 320AH discharge. This, of course, assumes a new battery with full capacity. After 3 years of hard use, I'd be tempted to derate capacity by 10% or 400 - 40 = 360AH. Discharging 80%, 360 x 80% = 288AH estimated usable capacity.
With the FNDC enabled,, the web application on the Mate shows a "snapshot" of Net Battery AH on the Summary page. If you're up with the chickens, before the PV Arrays/Charge Controllers wake up, you can get a quick read on consumption since last fully charged. (It will be a negative number.) If its in the -275 to -300 AH range, you can be sure the battery is being drawn down heavily by loads.
450AH battery, discharged a net -75AH since fully charged today.