ok my approach is not very scientific,, I have alot of arrows,, so I just try different ones till i find one that shoots well,,
Im not good at bare shafting,, but I would recommend finding a fletched arrow that shoots well, ,and then bare shaft if needed,,
I have seen test kits with several spined arrows you could try,, they were wood,, but that is what I would shoot out of a wood bow anyway,,
even the way you hold your bow is gonna effect arrow flight,, if you are just starting,, it may take a while before your testing is consistant,, the better your form,, the wider range of spine you can shoot,, etc etc etc,,