best way to use coupons is you don't, you save up enough for something significant such as fuku deal, HOWEVER if you might have commitment issues and cant save up too much there are few events that might work such as great plates (only 3.6k cp just enough to get the 500 keys reward), konoha tree (for low amounts that a bit so-so but still) not everytime but sometimes lucky star might be worth, depending on what is on the wheel, if there are assist pots or whatnot (but never go for the ninja in the wheel, is not worth it and almost never drops) and fuku shop if you manage to get a 50% discount card in the wheel (use only free attempts unless you want something very very bad and cant hold it till next time or so) then you can get ninjas or items at half price (right now kakuzu akatsuki is around 5k or so i think with the discount) and might be few others but dont remember right now, when you go for a wheel or rng event, don't go for whats too rng that you think it "might" drop cuz mostly it will not

usually is 140 free keys + 360 bought keys to make it 500, as i said i mainly started doing it for the breakthrough materials and the ninjas in the packs was extra bonuses, right now i think is not extremely great but in the past the IV and V contained things like kabuto sage, sasuke rinne and madara edo, alongside things like kakashi summer and whatnot

one pack gives this:

but i usually hold them till i get enough for the ninja i want since we might get fragments for them else where or so

don't go more then 500 cuz you don't have a reason why

fireworks is not so great to spend cp in it tho, missions is ok also dont forget to use a friend/alt to send yourself missions to get the bonus packs (i seen someone getting 50 edo madara frags from those)