Fins on small shortboards

General Discussion
1
I just shaped and glassed a 5'8" x 18 1/8" x 2 1/8" for a friend whose 2 other boards were a 5'10" Lost SDII and a 5'6" JC. I was going to try to discern fin positions from them and use a rough average of the two. They both have 1/4" toe so I was going to use that, but the back fin positions are very different on the two boards. The Lost's back fin is set around 3.25" roughly. Can't remember off the top of my head. And the JC was around 2 7/8". I loved the fin positions chart that surfding posted but unfortunately it doesn't cover boards any shorter than 6'! Does anyone have advice on smaller boards? The two other boards he has now are rounded square tails, and the 5'8" is a rounded pintail.
He's 5'5" and 125 lbs dripping wet so the board should be perfect for him as the JC is way too small and the Lost a bit too big. I'm using ProBox too so I have some movement, I'm just not sure if I should make it so that he can start around 3" and go up from there, or move it slightly higher up. Side fins will be helpful too, although I want to say both his boards are about the same.
Thanks!
2

Personally I would go with 3.25". But you can also split the diff between the two and go 3.06".  Further up (tighter cluster) for a more pivotal feel, further back for more drive. Your choice of fins, especially base length will make a difference. Fin sets with longer bases can be clustered a bit tighter. He will probably do ok with 4-4.25" base fins. Any thruster can be loosened up with a smaller center. Its good policy to have more than one center fin choice. Good luck!