OK, I worked it out and that formula is right. I just tried:
Rtop = 3280, Rbottom (test) = 2150
Vin = 5V, SensorValue = 357
Rbottom = (1.72 * 3280)/(5 - 1.74) = 1750 Ohms, which is wrong.
OK, I'm not sure I'm covering all aspects. I'm using 2 multimeters here, neither of which can read Amperage.