Does Fiora legend doubling apply when Fiora has negative Might from Punch First?

Doubling is specially defined to only increase, so it does nothing when her Might is negative. In this case she ends up at 4 might after Punch First. Negative might is treated as 0 for many effects, but effects that calculate a change to might — and doubling in particular — use the actual value where relevant (rule 143.2.b.1).