fins position on Evil twin

[sailpower]

Hi all,

I have an Evil 74 since July and 10 sessions with this board in many different conditions. When my board was delivered I didn't know exactly which position was the best for fins then I positioned fins in front of the box. I wonder what the best position is and what the influence of the fins position is in the behavior of the board.
Another question..
I weigh 80kg and I would be interested to upgrade the evil with MFC twin fins which are finer than starboard. Have you ever tryied it on evil? Which size can I buy for 3.7 and 4.2 sails and maybe 5.0?