Yeah. As mixture is ignited from two directions, it will combust quicker. This means that pressure will raise in more distinct manner and that you don't need as much advance in order to "wait for pressure to build up". This is also one of main reasons for twin plugs making more power: you can time the ignition so pressure spends more time pushing the piston down then before.
Correct readings should be found on dyno as it will differ from car to car. I suggest retarding it 5-6 degrees, then inching forward while measuring torque. When torque starts to drop off (or you expirience pre-ignition), it's time to back off.
Thank you for your time,