Free ARC Protractor

If you want to print a custom ARC protractor for your tonearm there is a wonderful link on Vinyl Engine to Conrad Hoffman's ARC Template Generator.It is a windows program and is small and excellent. You can specify any Pivot to Spindle distance and any Inner and Outergroove Radius based on your desire and get a printable ARC with a calibrated line to make sure the ARC is printed to Scale. I recommend an Ink Jet printer since if you print on heavy stock paper a laser printer will curl the paper.
Many laser printers (like my trusty HP 2100 that is still chugging along at 11 years in service) have an option for opening the back to allow a straight path for heavy stock.
True but for those that don't....

What is great also is that there are several different programs. You can run all sorts of comparisons for different Null Points and what is the effect when you deviate from the prescribed overhang. It shows changes in distortion etc.

In a Lofgren A setup a deviation of .5mm in extra overhang length can give you an approximation of a Lofgren B setup. You can also see that shortening in some situations the overhang (accidentally) can be worse than going longer.

You are able to plot maximum and average distortion based on differing setups.

You are also able to compare the difference between 9 inch, 10 inch and longer tonearms to see the effect on distortion.
Good site this Vinyl Engine, even the 'know it alls' could learn a few things there.

JAS and company should be commended more often