I might strange indeed, makes one wonder.
If you do get the vac can hooked up you can measure its total added advance using a timing light. Trick at idle is to get full extent of the canister rod to pull full advance and not start the mechanical advance addition via rpm increase. Also if the engine vac is less than 14 at idle you may need a vacuum pump, most vacuum cans dont pin out until around 12-13 " hg
The point is if you have mech adv and vac advance at idle you cant always count on mech advance being steady one way or the other at lower rpms, making the vac adv determination tricky so just try and keep mechanical adv out of the equation.
Otherwise the skip my morning coffee method is to spin the RPM to 4000+ and take a BTDC timing reading without and with the vacuum advance applied, a vacuum pump is good plan here.
All this needs is a good vacuum gage, dial back light or marked balancer to 60 BTDC and someone to hold the brake