A seat position ahead or behind that will not move the CG enough to matter.
You are wrong and you don't even refer to type when you make your statement.
I can send a copy to you as well, and you can have a play with the seat position?
You might be surprised how substantially you can see the CG move back and forth in the graphical representation for a standard Cherokee, especially with two heavy persons upfront. Something to think about if you want to do spin training.
Quote from Piper Aircraft Corporation Service Bulletin 753:
The pilot and co-pilot seat location of 85.5” as specified in the A.F.M.
2 is the centre position of the seat track. This may vary from a full forward position arm of 80.5” to 90.5” at the most rearward position. Each hole from the centre position of 85.5” changes the arm of the seat location 1.25”.
The seat position to be used for spins should be determined and the correct arm should be used in calculating the aircraft weight and centre of gravity prior to beginning the flight.
80.5" to 90.5" is what I use, and yes it does matter!