PVP Reduction IS based on a sliding scale. Here are my figures:

Lvl Damage PvP Dam PvP Reduction

Burst of Fire (5) 15 13 0.8667

Inv. Light. (5) 25 21 0.84

Ignite (9) 37 32 0.8649

Careless
Light. (19) 99 77 0.7778

Combust (29) 182 123 0.6759

Light.
Strike (34) 184 125 0.679

Firestrike (39) 302 190 0.629

Starfire (49) 612 385 0.629

Wildfire (59) 1024 645 0.629

Tudamorf has reported the PvP reduction at the high end to be 62.5%, which is within the round-off error of my figured.