Knight/Warlock
If you play with Leather, all your % dex from Gear is useless, because the % dex is added after the conversion. Can you add the % dex and % sta from gear first before you do the conversion mathematics? Of course buffs outside gear like Hero Potion, Zodiac Pets should be excluded because otherwise Knight/Warlock would have too much physical attack.
If doing the separation between %attribute from Gear and outside Gear is impossible/hard I suggest to consider all forms of % buffs before conversion and to increase the -35% atk nerf to 40% or higher. This would allows Knight/Warlock to have same physical attack as before but make Leather variants equal to Chain variants.
Also I do not know if it is in the devs interest to encourage these hybrid gears. Maybe Defense or LP should also boost the conversion. I think the best situation is if pure Leather, pure Tank and pure Chain are equally strong without weird gear combinations and stats.