Suggestion : [(Eco/4) +0] cap : 60 eco
The maximum number of chest would go 8->15 but the formula forces to invest to get rewards
Since the retro-woe will occur once a week only I think it is a good idea to make guilds having to invest instead of getting a flat minimum number of chests
Flat minimum = Big guild X breaks emp at 5 min before the end of the woe but don't really care about that retro woe and they get 4 chests * 7 = 28 per week without much time/effort investment.
No flat minimum = Big guild X have nothing to win unless they invest. Having invested gives an incentive to then defend the castle. Much better balance.