[ROVERNET - UK] Ignition timing on the 3500S (Federal)

[peter king]

I'm about to do a tune-up, and reading the manual on Ignition timing, I 
see two settings, one for 10.5:1 compression ratio and one for 8.5:1 
compression ratio. My owner's manual says engines in "some export 
markets" have 8.5:1. Any insight on this, or more to the point: set 
timing at TDC or 6° BTDC?

Thanks,

Peter