You want to derive the pivot to STYLUS distance which is the tonearm's effective length.
My guess is that the numbers you are quoting are the pivot to SPINDLE distance which is a smaller number - smaller by the amount of overhang.
In terms of real-world numbers (e.g. a Rega tonearm), a 239mm effective length (pivot to stylus) will yield a 222mm pivot to spindle distance.
The shorter 9" tonearms are in the 211mm range for pivot to spindle.
For our purposes, you can derive the stylus position as being a point that's 10mm ahead of the cartridge mounting bolts (nominal specification).
So, find the midpoint of the two cartridge mounting slots and draw an imaginary line connecting them.
From the midpoint of this line, draw another one (parallel to the cartridge mounting slots) that's 10 mm long.
This is where your average stylus would be positioned if you mounted your cartridge in the middle of the headshell slots.
Now, measure the straight-line distance from this point to the center of your arm's bearing.
This is your pivot to STYLUS distance which is referred to as EFFECTIVE LENGTH.
Run this number through John Ellison's way-cool Baerwaald / Lofgren spreadsheet (available throuth the Enjoy the music website - or ... a link on my support page). You'll get both the overhang (and by inference the pivot to spindle distance), as well as the headshell offset angle.
Thom @ Galibier