In reality optimal tail current will depend on both the bulk voltage, bulk current and probably cell temperature.
Equally important to LFP and lead-acid both, but ESPECIALLY dangerous with lead-acid if you never go to float and stay in CV forever due to thermal runaway.
Ideally, you follow the manufacturer's recommendation on what C/n rate at which to trigger the switch to float.
AND - crucially, if you cannot reach that stated value, but instead observe a "stall" or no further improvement in the tail-current drop say from 10-30 minutes, you switch to float.
Not watching for the "stall" has destroyed banks, because unless the cells are factory new when the manufacturers guidance was pretty easy to obtain, age and use increase the internal resistance, and the wise battery manager guy will observe say 6 months to a year later if a stall is occuring, note the amperage, and change the new amperage trigger for float a bit beyond that.
Thermal runaway lead-acid:
1) Battery stays in CV and never gets trigger to go into float.
2) At high CV, and very low current, this last part of the phase is very inefficient chemically.
3) If left on too long, that inefficiency turns into heat. You can't feel it at first, but give it a week or so. If you could actually feel the plates themselves, you'd know quicker.
4) When lead-acid gets warmer, the chemical reaction itself is more efficient.
5) Even though the battery is already nearly totally charged, this lingering heat increases the battery's demand for more current. But it is already charged!
6) Charger obeys, more unnecessary charging current flows into an already charged battery, creating more heat, creating more demand... you see where this is going.
For a consumer, usually this is in the form of bulged out gels or agms, tops of cases popped, vents blowing, terminals bending off at angles.
In a commercial environment, lets just say you don't want to drive up to your vault doors blown wide open, halon going off, firemen on the scene ...
All this from just a weeny little tail current. Just saying - don't discount the damage just a "little" current can do.