I believe it is more reasonable to regulate float current, not float volts.
Someone here told me it is reasonable for the battery to absorb .01C charge rate (100 hour charge current) without losing any water.. I would run it lower than that, at a C/200 rate.
As the battery temperature changes the float voltage will change of course but this wont change the amps.
Trying to find the right float voltage is fundamentally backwards in my opinion.
If you don't want to go through the trouble of setting up a constant current system, then hook up your battery to a laboratory power supply, set the amps to c/200, put it in a box with the rest of the system to keep the temperature the same, then come back in a week and read the voltage across the battery and set that voltage in your system.