How to??

You might have a CAP distributor, they had a bunch of advance, and base time was very retarded (-5).
They also had a vacuum amplifier, different vacuum advance. Might want to check the bottom of distributor cam part with slots. The number there, with an "R" is the advance in distributor degrees, 2x is crank degrees.

You can often find distributor timing specs in FSM, they vary by engine, w/ w/o CAP, trans and carb. The specs are in distributor degrees, same with test RPM, so you will need to calculate back to crank RPM and degrees. They are good starting points.