Garmin G1000 Pilot's Manual page 175

Altitude Constraint
7) Turn the small FMS Knob to enter the desired VNAV altitude.
Altitude Label
8) Press the ENT Key. The option to select MSL or AGL is now displayed.
9) Turn the small FMS Knob to select 'MSL' or 'AGL'.
10) Press the ENT Key. The cursor is placed in the 'VNAV' offset distance field.
190-00498-01 Rev. A
Figure 5-36 Entering Altitude Constraint
Figure 5-37 Selecting Altitude Mode
Figure 5-38 Entering Offset Value
Garmin G1000 Pilot's Guide for the Cessna Nav III
GPS NAVIGATION
Offset Value
5-39