Mounting of a tonearm.

I have an Oracle turntable with a rare Micro Seiki TA-1 tonearm. I don't have any dimensions or instructions for the installation of the tonearm.
How critical is the measurement from the spindle to the tonearm.?
How would i be able to determine the correct measurement ?

The tonearm sounds great the way it is installed but on some recordings i hear a little distortion on the last track and i am wondering if this dimension would have anything to do with it.
Thanks
Todd