I think it's worth a little more than one pawn 
I guess you mean it's value is increased by one pawn.
Some old "rule" I can't quite remember. Something like a knight outposted on the 5th rank is worth 4-5 points. A knight on the 6th is 5-6 points. Something like that.
Umm... that's really all I can think of. These sorts of rule always have exceptions though. A knight outposted on the 6th but on the queenside when all the action is on the kingside is worthless. The thing to keep in mind is mobility / activity is what makes pieces valuable.
If a Rook on the 7th Rank is worth one pawn
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There is the saying that a rook on the 7th rank is worth 1 pawn. What about other positional improvements.
Is there a way to roughly estimate your positional advantage in terms of +#? For example it may be the case that certain improvements are always worth +.5 in similar positions, etc.