Engine Op Mode

Can anyone answer when the ECM is looking up which operating mode to be based on air density. Does it look at just barometric pressure or does it reference what the turbo charger is doing? What I am getting at is if I can just use 1 table up to .4 then just have it switch over to a high altitude table. I dont want to do that if it would be to harsh of a change switching back and forth once I am building boost. I tow over mountain passes and want to prepare as best as possible. This is on an 06 Cummins in HP Tuners.