I saw this same formula earlier and it's not quite right because guild rank and dual element bonuses stack. You add the armor stat and the level bonus then multiply the sum by your guild rank bonus and then multiply that result by your element bonus (if you have two element bonuses add the percentages and then multiply).
When I checked this against a mono armor it came out exactly right. When I tried it with dual elements it came out within 1-2 points (I believe the discrepancy is because it rounds each multiplication result up as I was always 1-2 points higher than the expected result).