Glad someone is showing interest in this rather than jumping on that decorative solid state thing shown in my avatar.
My understanding is this is how much the ballast will drive the lamp and in terms of lumen output. When looking for a replacement ballast, what ballast factor should I use? How does it effect lamp life? Energy efficiency?
The lamps are supposed to give you the rated life between 0.7 to 1.2 BF range.
Short answer. If you're replacing just a ballast or two, you use the N. For service stocks, you only need two types. 120-277 2N and 4N. These will cover for 17-32W T8 on 1 to 4 lamp applications. Most ballasts are approved to operate one less lamp than rated. You wouldn't be able to measure the efficiency difference for the system as a whole if you're spot replacing. If you're replacing a large quantity, you select carefully.
For high bays, you'll need the H ballasts or else you lose about 25% of output.
Your understanding is correct. You take the mean or design lamp lumen and multiply it by the BF. You tune the system by messing with lamp quantity , lamp type and ballast factor. BF isn't free lunch. If you lower or raise the output, the power use follows. Some ballasts shifts in BF a bit if you change the lamp type.
If your application needs and existing ballasts permit it, lamp tuning has a very quick payback and it will run laps around LE Decoras. Sometimes it's just a matter of using different lamps.