The G84 tapping cycle is calculated like this
G84 Depth in Z, Start point, Thread pitch * rpm
Tap is 20 tpi so 1/20=.050"
rpm=100
So feed = .050*100 = 5.0 (F5.0)
G84 Z-.75 R.100 F5.
As for cutting speeds
Aluminium 70-90 ft/min (21-27m/min)
Brass 80-100 (24-30)
Bronze 30-40 (9-12)
Iron 15-30 (4.5-9)
Mild steel 20-40 (9-15)
Steel Alloys 15-25 (4.5-8)
Stainless 10-15 (3-4.5)
BSPT and NPT taps should run at 50-60% of the above figures.
I hope this helps
Graham.