The FMS will base its calculations on the last time something was EXECuted. Any calculations the FMS makes are based on the data entered into the RTE and PERF INIT pages, so in theory, the calculations should get more accurate every time something is EXECuted. For example, if you execute a flight plan change in the climb, the calculated T/C point will be updated based on the aircraft's present performance.
Mark - Just Flight