The use of IAS or TAS to calculate a Vertical Speed to achieve a specific gradient, has no place in the flight deck. Although performance planners would use them to calculate curved departure splays.
Groundspeed times required gradient (%) equals minimum required VS.
If you think about flying a -3deg approach, a rule of thumb is to half your groundspeed, add a zero and add 50.
The effect of wind,temperature,pressure and instrument errors relegate IAS/TAS to the performance planners.
I don't have a copy with me of "Handling the Big Jets" or "Aerodynamics for Naval Aviators" however I am sure it would have a reference.
