The "Manufacturing Planning and Estamating Handbook on page 6-56 has a table 6-20.
T is in minutes and C is the calculated constant.
V is in SFM.
The table gives the n value to use.
Work Material, Sinistered Carbide, High-speed steel.
Steel, soft .20 (this value using 300 FPM), .12
Steel, hardened .25 , .15
Cast iron .25 , .15
Light metals .41 , .41

The table gives those values for a 60 minute tool life, .009 ipr and .062 depth of cut for HSS.
Carbide using .010 ipr and .100 depth of cut.

For guesstimation I have just used .25 for Carbide cutters and .1 for HSS, and what ever FPM and IPR and the apparent tool life I was getting in minutes and change T to what I want to change for a guess at a new FPM