I'm not sure what AMS do in mass de-icing, but at LHR we activate a functionality where;
TOBT is the time the aircraft is ready to de-ice.
EEZT (estimated duration of de-ice) is also supplied by ground handlers.
Then the TOBT after de-icing is the sum of these times, which is then used for TSAT calculation.
The idea is that the TSAT is published before de-icing actualy starts, and is locked once the aircraft calls ready to de-ice. This means everyone knows if it's feasible to de-ice and depart in the time allowed by the TSAT/TTOT before committing to de-icing.