fin configuration on Gun 7'2" to 7'8"

Hi everyone,  first off, an intro as this is my first post.

I'm a West Aussie who has lazily been developing boards for the past 25 years.  I started off developing hollow balsa windsurfers then used surfboards to continue my development.  Built around 70 boards all for myself as I wouldn't want to sell anything till I got it right. Just a hobby but I'm ok at what I do.

So I'm looking a building a 7'2" or 7'8" gun for down south waves. Looking at dragging it out at about 3~4 head high.  It will be a low volume (for a board of that size) pinny gun with a flat tail as this is what I have been riding in my 6'8" rounded pin.  My current fin configuration is a thruster and its well balanced.  The flat tail means I am slightly further back than most surfers and tend to ride with my back foot over the sweet spot the whole time.  I'm only 70kg so when I put the normal tail kick in my boards I end up to far forward and have to move back to hit the sweet spot when I need it.  So with this design I am worried about having too much board out in front of me.  So I'm wondering how far to move the fins forward from the tail if anything at all.  I will be sticking a few extra fin plugs in the board and I can always make some custom fins up if I get it wrong.

Any advice out there from people who build similar boards for similar conditions.

Cheers

Woodyads.