To do the “instillation value” without vacuum. Does that mean you pull and plug the vacuum hose connected to the vacuum advance? Then adjust timing to 7 degrees?

I’ve got a 1974 450SL and the chart shows 7 degrees btdc.

When I pull the vacuum from the vacuum advance on the distributor the timing mark changes to about 20 degrees btdc? Is this correct?

Or should I set the timing to 5 degrees atdc with vacuum hose attached THEN unplug the vacuum line from the vacuum advance and plug the line. Then adjust to 7 degrees?

What is the best timing for this model. It runs at 5 degrees atdc. But like people say it runs better at btdc.

Also when I use my multimeter turned to dwell reading it just shows 45 degrees.. it doesn’t move. Even my timing light has the ability to read dwell but also says 45 degrees. It doesn’t change. Am I doing something wrong?

Thanks
Josh