[Aegis]

Probably the best way I think is to have them be relative. If Player A has PO-Level 1, Player B has PO-Level 3, and Player C has PO-Level 0, then for straight diplomacy:
A has level 0 to B and level 1 to C
B has level 2 to A and level 3 to C
C has level 0 to both A and B

Consequently, if A asks C to go to war with B, then his request gets upped one level of importance for his PO-level, and dropped three levels of importance due to B's PO-level, so his request is at -2.

-Aegis