If I am attacking into Fiora - Peerless while she is on Forbidding Waste, does Forbidding Waste apply before her Double Might?

No, Forbidding Waste's -2 applies after Fiora's doubling in the arithmetic layer, resulting in 4 Might, not 2.

Here's why: The Arithmetic Layer system (Rule 472.3.d) mandates that all increases to Might are applied first, and all decreases are applied last — regardless of when the effects became active. Fiora's doubling creates a +3 increase (equal to her base Might of 3 per Rule 432.1), and Forbidding Waste applies a -2 decrease. Increases first: 3 + 3 = 6. Decreases last: 6 − 2 = 4.

This is the same rule ordering used for Fiora, Victorious on Forbidding Wastes (FAQ #10840) and consistent with how doubling interacts with equipment/passive bonuses (FAQ #10971) — any active increases are factored into the doubled value before subtractive effects apply.