Why rigid tapping feed rate is not a free choice
Unlike milling, where feed rate and spindle speed can be adjusted somewhat independently, rigid tapping locks feed rate and RPM together by the physical pitch of the thread itself. The tap has to advance exactly one thread pitch per revolution of the spindle, since that is how a thread physically works, which means feed rate is not something to optimize for chip load the way it is in milling.
How the number is calculated
Tapping feed rate in inches per minute equals spindle speed in RPM divided by threads per inch, since dividing by TPI converts revolutions per minute directly into the linear travel per minute that matches one thread pitch per revolution.
Practical tips
Because feed and speed are locked together this way, the only real variable left to choose when rigid tapping is spindle RPM itself, selected based on the tap manufacturer's recommended surface speed for the material being tapped. Make sure the control is actually running in a true rigid tapping mode with encoder-synchronized spindle and Z motion, since running this feed and speed combination without rigid tap synchronization will strip or break the tap almost immediately.