Do not use ticket on anything older than 24th main/mini box.
Each time 50% off sales comes all previous boxes + increment of 8 are sold at half price.
This leaves only 24th-31st main/mini box to use UR/SR Dream tickets on.
No point to level up all characters to 45. I am a late game player and I don't need to upgrade my deck anymore. Already saved up 3000 gems since the end of the previous 50% discount.