Question about how i-Level is Calculated

[Docent]

Sum of all item level.
Divide by 13.

For jobs with no off-hand, multiply main hand item level by 2.

In your example:

100 - Main Hand (200 item levels)
100 - Head
110 - Body
100 - Hands
100 - Waist
120 - Legs
100 - Feet
100 - Neckless
100 - Earrings
100 - Bracelet
100 - Ring
120 - Ring

= ( 200 + 100 + 110 + 100 + 100 + 120 + 100 + 100 + 100 + 100 + 100 + 120 ) / 13
= 1350 / 13
= 103.846153~

Value is floored (everything is ignored after the decimal) by the game, so you need 1352 total item levels for it to read as 104.

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In other words, the majority of the time you go up "10" item level on a piece of gear, you should gain 1 average item level (10 out of 13 times). Two-handed weapons, of course, are worth 20 points, so will always gain 1 levels, with a decent chance of giving 2.