Which factors decide the amount of spacing at which a ligature is divided

Hello,

at first I hope I'm using the wright english words to make the problem understandable, if not please ask.

InDesign decides at a certain letter spacing to divide a ligature into his original Letters. This certain amount of spacing is different at every font. So which factors decide this certain amount of letterspacing at which the ligatures are divided? (There must be factors, otherwise it would be the same amount of letter spacing at every font) I'm creating fonts so its important for me to know the parameters InDesign uses to calculate this amount of letterspacing.

Thank you very much in advance!

Aaron