The feed rate (in inches per revolution) is simply 1 divided by the number of threads per inch.
A 5/8-18 tap requires a feedrate of 1/18 or .0556 ipr.
If you are programming in inches per minute, the feedrate will be the spindle rpm divided by # of threads per inch.
For example, at 360 rpm, the feedrate will be 20 ipm (360/18).
I assume that your control supports rigid tapping