I've added 6 batteries to the original 1st one over the last few years. I've stabilized on a 49.5v cut-off so every morning the battery bank is ~50v for a few hours until PV kicks in.
When I build my new batteries I build them with this in mind so they are ~50v when I assemble them. Then, it's just a matter of picking any morning to add the new battery to the powerwall. It doesn't matter if one is 49.8v and the other is 50.2v - it just needs to be 'fairly close'.
My powerwall is lithium-ion so matching voltages is easy. If ou're doing LifePo4 maybe you use SOC and voltage since it's a flatter curve. My point is you might find you add a 3rd and 4th and 5th and ... battery over time and working out a repeatable approach can reduce stress and add safety.
