Henry,
FIRST
What Dan_Ed said.
SECOND
My TT spindle has a lathe mark that's dead center. Easy to see and measure precisely.
Your spindle doesn't have a center mark? You can make your own - for free.
How? Simple. Mark a dot, as tiny and precise as you please, on a square of Scotch Tape. Stick that on your spindle and spin the platter. If the dot oscillates it's not centered. Move the tape, spin again, reiterate until the dot remains stationary. Voila!
(It took me longer to type that than it takes to actually do it. It's so easy it's trivial.)
THREE
Your tonearm pivot point is "ill-defined"? No problem. Use the same trick. I've done this with Rega and OL arms and it should work with most arms where the bearing center isn't obvious or accessible to a ruler. Again, so easy it's trivial.
FOUR
Your ruler deflects over a 9-12" span? Even I'm not THAT cheap. Buy a sturdier ruler.
As I said, I've used a Feickert and I agree it's well made and measures S2P quite accurately. However, I can and do perform the same task with similar accuracy for 99% less money. So can anyone with a little thought and care. It's not the cost of the tools, it's how you use them.