You're exactly my size. For wave sailing on the K86, I use the back option for the back strap and then place the front strap where it's comfortable. Normally I end up at around 44cms between the back hole on the front strap to the front hole on the back strap (I don't remember what front setting that strap setting that corresponds to on the K86).
I place the mast foot at the middle of the track and don't move it even with small sails. Sometimes I tune it at most +-1cm to suit the conditions. Forwards in more chop etc.
I place the fin a bit forwards in the box. There is some fine tuning that one might make to make the board sit well in the turn, but my standard position for wave is to place the middle of the fin right under the rear foot strap screw.
The best tuning you can do for smaller sail in waves is to get an extra smaller fin. The Drake Natural fin is very good, but for 5.0 I think you should look at something like the 22cm. for this board.