Average item Lv Formula?

[Ronyx]

Im a summoner. No Shields and Im using a Crimson Circlet, soo no Cowl. Lets see 12 Slots Total.
Crimson Lv80
Darklight Tome 55
Vanya Robe 70
Darklights x 7
Austere tights 60
Demagogue Ring 60
Total 815 / 12 = 67.9
Game says item Avg Lv 66