That's exactly how I do it. I tap the button to lock out however many gears I would like. I almost always have 6th locked out...if flat or long, generally downhill grades. I've locked out 5th on 6% or greater climbs.
On our weekend trip last week, there is a 10% climb that you have to start from a stop. I put it in M and 1st gear until there was a slight dip for a second. This dip allowed me to get a little speed and then I manually went to second for the rest of the climb. I could have put the pedal down and muscled up more aggressively, but I did what I felt was easiest for the truck.
As Tystevens said, you'll have it figured in short order.