You're on the right path.
Note that you can skip the conversion to seconds since both RPM and feed are in minutes.
Feed Speed/Spindle Speed = Feed per Rev (FPR)
100/1680 = 0.059 Feed per Rev
The machine is moving 0.059 forward for each revolution of the cutter.
Now that you know how much material is removed in one revolution you divide by the number of teeth in your cutter.
0.059/2 = .0295
This is the amount of material removed by each tooth as it swipes through the material. This is feed per tooth (FPT) also know as chip load.
So if you have 4 teeth you can feed twice as fast and have the same load per tooth.
200 Feed / 1680 RPM = 0.119 Feed per Rev
0.119 Feed per Rev / 4 Teeth = .029 Feed per Tooth
Bob
You can always spot the pioneers -- They're the ones with the arrows in their backs.