How many characters fit on a line
The rule everyone follows is two lines of around forty characters, held on screen long enough to read without rushing: between one and seven seconds, and never less than one. It isn't our rule — it's what settled in television and film over decades, and players assume it's being followed.
The segments come out cut on that basis, not on loose sentences. If you edit the text afterwards to fix a name, the cut stays put: what changes is what's read, not where each subtitle enters and leaves.