I don't think Scathe should be more potency than Fire I, especially if it has the effect of extending AF/UI.
Fire I is 180 potency, while Scathe has an 'effective' potency of 120 (100 +20% of 200).

You could maybe increase the potency to 150 (with a 20% chance of 200 still for an 'effective' potency of 160), or alternatively increase the chance of doubling to 50% or 60%.
That, along with a 3s refresh of AF/UI, would make it functional without disrupting the BLM rotation.