it's random and is somewhere between 635 and 715 for all the plates opened (that costs 700 coupons).

No, for f2p lucky board is always a better event to spend your coupons on.

In this event the best option you can hope for to invest your coupons is one try per day in the myoboku trial's wheel of offerings.

100 coupons spent there have a very high chance to grant you back a good reward (10 tactics pages, a level 5 rainbow magatama or 3 charm packs are not at all rare drops in that feature).