What should be the cut-off voltage for charging lead acid batteries through PV panel?
This is the tradeoff using a cheap charger with no float mode.  Capacity vs battery life.

Your options are ; 


Get a float mode when charge current drops low. 
Get more battery capacity. Now. Or...

Replace the battery sooner, than expected



If your charge periods are short term, use 14.4  if it charges up always more than you can store,   use 14.1

If there is no load during charge and only used at night, use 14.1, otherwise higher.