You will be looking for half scale which is 50% duty cycle. For example. For a four cylinder, max scale will be 90 degrees. This means that 45 degrees on the four cylinder scale will be 50% duty cycle.

I actually dug out an old Dwell meter and experimented with it. It was reading exactly center scale on my 300E pin 3. I didn't know whether or not to trust it at the time, so I bought the $27 meter. It confirmed that my lambda was set at 50%. This gave me confidence in using the dwell meter, but now I had the duty cycle meter so I didn't need it.

Hope this helps,