According to my investigation on my Featherino 1-1 and Olbec 1-1 the formula is
Moonrise Strike damage = base damage / 1000 x (attack - defense) + base damage (round down)
with base damage = 50% of max damage
From my Paeia 1-1 I could derive nearly the same formula for the Moonrise Ward:
Moonrise Ward damage reduction = - base damage reduction / 1000 x (attack - defense) + base damage reduction (probably round down)
with base damage reduction = 50% x max damage reduction
The formula looks quite simple and adaptable to any owl hero, so I give it high chances that it is correct.
The data for Fulvia seems to fit with it as well.