Hi Rene,
You didn't miss anything. You are correct that there is no preferred effective length (as long as you are not using an arc style protractor which is drawn for one and only one set of parameters).
The benefit of a two point protractor is that with enough diligence, flexibility in the headshell slots, etc. you can get a perfect alignment irrespective of your pivot to spindle distance being off - as long as you have enough movement in the headshell slots to adjust the effective length and offset angle to match your "wrong" pivot to spindle distance.
With an arc style protractor, the price you pay for ease of visualizing and solving the problem is that the pivot to spindle distance must be very close to perfect, because the arc is drawn from the theoretically correct arm mounting (pivot) point.
Dsa - A few posts up, I have a link to an arc protractor thread. In that thread, Richard posted a link to the protractor fellow. He loves it even more than his "original" arc protractor.
Cheers,
Thom
You didn't miss anything. You are correct that there is no preferred effective length (as long as you are not using an arc style protractor which is drawn for one and only one set of parameters).
The benefit of a two point protractor is that with enough diligence, flexibility in the headshell slots, etc. you can get a perfect alignment irrespective of your pivot to spindle distance being off - as long as you have enough movement in the headshell slots to adjust the effective length and offset angle to match your "wrong" pivot to spindle distance.
With an arc style protractor, the price you pay for ease of visualizing and solving the problem is that the pivot to spindle distance must be very close to perfect, because the arc is drawn from the theoretically correct arm mounting (pivot) point.
Dsa - A few posts up, I have a link to an arc protractor thread. In that thread, Richard posted a link to the protractor fellow. He loves it even more than his "original" arc protractor.
Cheers,
Thom