2 Fitting Questions........

hi, wondering if there was any bike fitting technique to know how far
forward/back my seat should be clamped on the rails (ie. is there a
certain distance the tip should be behind the bottom bracket?), or is
it just personal preference?

also, is there a fitting technique regarding stem length selection (ie.
a specific angle your elbows should bend when on the hoods, drops,
etc), or again just personal preference?

thanks!