To cut through the other stuff, Lohanimal, the statement you quoted does not indicate that one MUST use an FR headshell with an FR tonearm. Is that what you meant to say? Your quote from the owners manual only would stipulate that IF one is using an FR headshell, THEN one should place the stylus tip 7mm behind the front edge of the headshell. This helpful hint does not prohibit the use of other headshells, so long as they properly fit the bayonet mount. If using some other brand, one would simply need an alignment gauge for setting overhang. And anyway, it would be at least as much of a pain in the ass to measure that 7mm as to use a proper alignment gauge, maybe worse.
As to accuracy, I think the best most of us can do is within 1mm net error (or +/-0.5mm, in other words), between setting P2S and overhang. If you get it that close, you've done well. Then there's the longitudinal angle of the cantilever with respect to the particular algorithm. That requires patience.