Chain Brake and RPMs

[trails]

I am trying to tune the low idle on my stihl 440 magnum. This saw has the factory limiters on the screws. I have clean filter, fresh fuel 50:1, clean spark plug, normal chain tension, new bar and chain. I have the low at 1/4 turn out as recommended on the saw (High at 3/4 turn out as recommended (high clocks in at 12,700RPM) I read 440 can go to 13,500 but better low than high for now) Using a tachometer to guide me, I adjust the LA screw till the RPMs read 2500-2600. the reading is a bit jumpy. Then I put the chain brake on and they drop to 2100, 1900, 2200. It even died twice when I put the brake on. So I take the brake off, tune up to 3000, without causing the chain to skip mind you, then I put the brake on and it sits around +-2500RPM

1. How, mechanically does the chain brake slow the RPMs if there is not enough clutch rotation to engage the chain when the brake is disengaged. 
2. Is this normal? 
3. How do I close this rpm gap between brake on and brake off to achieve one single low idle reading.
4. If I cant close the gap, is it so bad to run the saw with a bit too high of a low idle? After all it can go up to 12-13,000RPM.

[donbj]

The brake band clamps down on the clutch drum to keep it from turning. The clutch must be grabbing the drum at idle speed and turning it for the brake to slow it/idle down. Do you have the bar and chain on when doing these adjustments?

[trails]

I see, I agree if the brake is making a difference in rpm there must be some rotation present for the brake to slow down. I do have the bar and chain on, normal chain tension i.e. I don't think the chain is so tight as to hold what would otherwise be a moving/running chain.