Yu-Gi-Oh! Duel Monsters II: Dark duel Stories
Your duelist level does not start at 80. (Deck Volume − 400) ÷ 7 + 15 reads like (400 - 400) / 7 + 15. 400 minus 400 is 0, meaning you're dividing by 0, which is impossible so we ignore the division entirely. You're left with the +15. 15 = duel level.