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.

THANKS GRAHAM