I could bore you with long, complicated case *ysis but....

How far exactly are you away form 1st place in term of % damage? If it's greater than 30%, then you will lose on coupons and only gain some coin/exp. If it's around 20%, you will about break even on coupon and gain some coin/exp. If it's less than 20%, then you will likely gain both coupon and coin/exp.

Also, depending on how long fox last in your server, you can consider using revive instead of buff. Generally speaking, revive is better if the fox lasts less than 17 min from start (i.e. end before 8:27).

Also, remember the buff is not all or nothing. If they are only, for example, 10% over you. 2 layers of buff for 20 coupon will do fine, and you will make a profit on coupons. (or alternatively, you could use revives, which will cost you 15-20 coupons for about the same boost in overall damage)