Nandric, in all alignment procedures (except the SME) it's mandatory that a prescribed stylus-to-pivot distance (sometimes called 'Effective Arm Length') be maintained (thus necessitating slotted headshells ;-)
Further, a fixed arm length requires a specific (for that tonearm) spindle to pivot distance, and most tonearms come with an appropriate (though not always well crafted) template. If the one you got is sloppy, or non-existant, it's best to make your own out of a piece of acrylic. Slip one end over the spindle (with the platter removed) and, through a tiny hole in the template drilled the correct distance from the center of the spindle, scribe or draw an arc on the arm board. You can drill the arm mounting hole(s) anywhere along that arc.
Dan asserts that "With an arc-style protractor made for the table/arm combo, there is no need to measure P2S" (I think he meant cartridge/arm combo,) but unfortunately, this is true only if the arm mounting holes have been drilled, and the arm mounted with utmost accuracy, the correct distance from the spindle, per the tonearm manufacturer's specs. In reality, that is REALLY HARD TO DO without proper shop tools, drill press, etc.) If there's any doubt (and even if your TT came with the TA mounted) it's best to field-measure your particular equipment setup and provide the actual P2S dimension to the toolmaker prior to fabrication ;-)
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