I use SetCPU as well but I am a little more aggressive.
ScreenOff = 300Mhz
Less than 50% Batt = 800Mhz
Less than 25% Batt = 600 Mhz
I also have it set to throttle down to 800Mhz when temp goes above 50C but thats not really about Battery Life.
You really don't notice the lowered CPU at all. Especially during phone calls or TXTing.
I think the real savings is in the screen off. My guess is the stock automatic scaling is not doing a good enough job on it's own when the phone is sleeping. and most of our battery life was being blown away in standby.
With those settings I have been able to go almost a day and a half on a charge with moderate use where before even light use would only get me 6-8 hours.
And that is with a BP6 not a 7!
With those settings as your battery goes lower the longer it will take to drain. It really was remarkable.