What Length Rail.

[fcr]

I am wondering what the logic is to determine the rail length of an AR upper. If it doesn't have F sight/gas block, finish just behind or beyond the low profile GB? I am putting together a 16" midlength gas. and just wanted to know the reasons to finish a free float rail long, short etc. . Is there a rule for function, does it depend on the extra you are going to run like lights and so on ? Thanks in advance.