Sounds like you're on track. It's really not as hard as people sometimes expect it to be.

You'll probably get best resuts by using "Medium" since you are focusing on the 70dB to 80dB range and that puts you right in the middle of the range.

You don't need to "zero" the meter - you just need to set it to a range that will let you see 75dB and adjust each channel's trim on the 990 to get a reading of 75dB.