Surfing Waves

So I picked up a new small wave board. Here's some stats if it helps you guys.

Me:
6'0
172lbs

Board:
7'0 Long
21 5/8 Wide
2 3/4 Thick
18 1/2 Round nose
15 Rounded pin tail
Simple single to a conservative double concave
4+1 Fin setup

I've been taking it out on small days, 2 feet on the regular, 4 footer every once in a while (it's been slow here). Tried as a 2+1 (GL sidebites, 6.5 Harbor center fin) and it felt fine, if a tad slow. Maybe slow isn't the right word. It felt secure. Like it had traction. Bottom turns felt pretty good in some of the better sets that came through.

The very next day I took it out as a quad (Kinetic X4) and there was a noticeable amount of looseness, which was fun, and definitely a lot more speed, also fun.

I was wondering if it's worth trying as a single fin for the small mush. If I'm planning on just trimming around the beach break will I be under-finned with a single 6.5? I also have a 8" True Ames Cali Classic. The quad was fast and fun but it doesn't glide. I was also concerned with the boards tail with a single fin. At 15 inches it's pretty narrow. Maybe I should just shut up and try both the 6.5 and the 8? All of this is to avoid having to run back to the car to swap out fins .

Just try them, without a video only you know what works for you, try the 6.5" at various positions in the centre box, repeat with the 8" .
I hope you have a box rather than plugs back equals tight forward equals loose.
Just do it. Go crazy experiment find things out,

Simple rule of thumb for Longboards ....... Inch for Foot. If you have a 9 foot board than use a 9 inch single fin. 9'6" single fin = 9.5" fin. When you add side bites you take away from the center. with GL's I would use a 7.5" to 8.0". With GX's I would say 7.0" or 6.5". Depending on your build you can move the scale up or down a bit by 1/2". And we're talking standard fins not hatchets or cutaways or D-fins, etc.

Also aways put a new fin in the center of the box. Too loose ? move it back a centimeter. Too Stiff ? move it forward just a centimeter. You be amazed that you 6.5" all the way to the back might act like you 8" all the way forward.

Oh, and who told you a 15" tail is "pretty narrow" ?