hey big boy !!

by the way loooved the images from the lake

fins are a funny thing u have to keep trying to get that combo right, not just for the board and sail but for the conditions. you may get some info from here but were all individuals and have totally different styles in riding n rigging size ect
look at the fins you have and evaluate what are they doing and what they aren't. the crap ones that dont do nothing for you, chop em rake them stiffen them up or fatten them up to give u an idea of what you want from them
fins make a MASSIVE difference to ride quality confidence can turn a pig of a board to an absolute legend!
for my money i would go C3, Delta are doing some good fins too, always by them bigger then u can chop them down :-)
Sam hase some nice C3 in at the mo cheep too
oh and sailing them at the lake in crap conditions will make everything feel **** you need to go test fins at a good spot with consistent wind and conditions that suit what your going for .. this is the most frustrating part of it as we live in the windless state of NSW.

have fun come round I'm playing with fins at the mo ;-) if I can help I will