You already have everything you need to set the Pivot-to-Spindle *and* align your cartridge.
1. Use your ruler to set the Pivot-to-Spindle. Take your time. Tighten the articulated armboard (tight, but not too tight).
But what if it isn't perfect? Don't worry, and proceed to Step #2.
2. Go the Conrad Hoffman website http://conradhoffman.com/chsw.htm and download his free cartridge setup protractor. Enter your arms spindle to pivot distance and print out a customer arc protractor template. Pull out that ruler and double check what you print out to make sure your scale is correct.
3. Carefully cut out the hole for the spindle, and begin to align the cartridge per the instructions that come along with Conrad's tool.
4. If your cartridge won't align perfectly at both points on the arc...guess what? Your Pivot-to-Spindle is beyond the acceptable margin of error, and you go back to Step 1.
There is only one Pivot-to-Spindle and one Effective Length that will allow the stylus to perfectly touch both points on the arc. If all align, you *know* it's set up correctly.
Read the README.TXT that comes along with Conrad's tool. It explains the process far better than I just did (and I tested and gave feedback on many of the early versions of this tool).