Mounting a Jelco Tonearm

This is from a good friend of mine who is having a dilemna mounting a Jelco Tonearm.  I hope someone is able to help:

" I've been trying to determine a foolproof method for accurately locating the hole for a tonearm in a plinth.   I think the pivot to spindle distance or the total effective length provided by Jelco is just wrong.    Everytime I mount this thing it seems like it's not quite in the position it should be so the cartridges sit 1) in the middle of the headshell and 2) line up nicely with the alignment grid.  I think one of the numbers is wrong. But which one, and by how much?

I actually have one of those Dennesen Soundtracktors.   Do you remember those?  They are basically an older, plastic version of the expensive, but very nice aluminum Dr. Feikert one.

There's got to be a simple, logical way to work this out.  

It's amazing I can't seem to find information on the Internet about this.  I wonder what audio shops do?"

Thanks in advance for your comments,

Louis