There is also the problem that the voltage while charging is higher than the rest voltage. And after you remove the charging current, voltage drops slowly. I'm designing a lead acid cell charger/conditioner/rescuer for Enersys Cyclon 2V 2.5Ah cells, and one of the specifications is that the charge percentage can be found by looking at the voltage 4 hours after any charge or discharge has been terminated.
In any case... a float charge is typically done after the cell or battery is mostly or fully charged. In that case, the float charge is applied as a constant -voltage-, compensated for temperature.
The continuous application of a constant current without limiting the voltage to the float charge voltage, will eventually ruin the cell or battery.
YMMV, but here is the datasheet for the lead acid cells I'm using, it should open to Page 10 which covers constant voltage charging and float charge:
http://www.datasheetlib.com/datasheet/1170821/0810-0004_enersys.html?page=10#datasheet