Can Ancient Henge's energy-to-power conversion reaction be used during Bullet Time's resolution to pay for Bullet Time's variable cost, or must it be tapped in advance?

Ruling: Ancient Henge can be used during Bullet Time's resolution to pay for the variable cost. You do not need to tap it in advance or declare how much energy you want to convert before resolution.

Sequence:

• Play Bullet Time
• During Bullet Time's resolution, when you need to pay the variable cost
• Use Ancient Henge's reaction ability to convert energy to power as needed

Nuances:

• This works because Ancient Henge is a reaction with an add ability, and it can be used during resolution when Bullet Time requires you to pay a cost
• This simplifies energy generation to work 'whenever needed' without keeping reaction windows in mind