Currently using
lvl 40 chain mail with 43dex

I was thinking to change it to lvl 50-55 ee armor
but how much ee i need to replace the 43 dex

cant really give you an exact number, but your 43 dex chain might be good enough depending on the group you are in (assuming you cata rather than solo) If your group has a tank then you can sit in the mid using your dex. If there isnt a tank in your game an armor with around 100EE will do just fine and you can sit in the front and tank.

Mystical dragon skull scale armor
Level Req: 55
Defense: 52 to 210

+142% Enhanced Effect
+6% Quick Draw
+2% Power Shot

should this do the trick?

plate mail gives more defense so i suggest use one

for ee armors, use heavy armors, the added defense is much greater.
many people prefer dex or str armors due to faster killing