Thrust Index is primarily calculated based on the airport elevation and outside air temperature, but the status of the cabin air and engine anti-icing must also be taken into effect. 

The Thrust Index required for a "Rated Thust" (full power) takeoff at the current airfield elevation and temperature will be set automatically when loading into the aircraft, or when the takeoff/landing speed chart is clicked.

A "Flexible Thrust" (reduced power) takeoff can be performed by setting the Thrust Index to a value lower than the value required for Rated Thrust. However, note that the throttles must always be at, or forward of, the detent position, and the thrust meters must indicate 100% thrust prior to reaching 60 knots in order to safely proceed with the takeoff. 

The full procedures for a "Rated Thrust" Takeoff and a "Flexible Thrust" Takeoff, as well as the Thrust Index tables, can be found in the F28 Professional Operation Manual.