Once you've established TDC, measure the circumference of your balancer. Divide that exact number by 10. THAT number is how far to measure from TDC to arrive at 36 degree's BTDC.
360/10=36 you see... Half of the measurment used to arrive at 36 would be 18 BTDC, etc. etc. That way you can verify your timing marks or add more marks because it's likely your timing "lines" on the damper stop before reaching 36. If your going to use vacuum advance as well, then carry forward some new lines out to about 55 degrees. Using tape or similiar on the damper face.
About 50-52 BTDC with base/mechanical and vacuum advance all in (10-15" of vacuum) is about right for a cruise setting and max mpg.
20 base with 18 mechanical advance on my 427. My vacuum advances to much and at to high a vacuum setpoint so I intend to replace it with an adjustable GM unit (for my "Ready to Run" MSD distributor).
Bear in mind: MY motor has a lot of "quench" designed into the combustion chamber/piston dome, I may be able to run more advance than the average 427. 38 is working very well, I suspect 40 would be "OK". In fact, I have about 7000 miles on my "new" motor now and it's always run at least 38, on occasion above 40 because I screwed up the timing a time or two. Run's great, keeps getting better with more miles on it!
