Calculating Base Resistor for Darlington Pair
You are, in one respect, correct in not trusting hfe, but not for the reason you think.

hfe is specified for a certain voltage and current. For the TIP120, this is a Vce of 3 volts. Problem is, you don't want to run your transistor at 3 volts. You want the transistor to be in saturation, with Vce as small as possible.

Looking at saturation numbers, you'll see a specified base current and collector current. As a good rule of thumb, use the closest numbers to your application. Then, you can compute your base resistor as you suggest.

When using single transistors, the general rule is to assume an hfe of 10 to drive the transistor into saturation. This is not a hard and fast rule, but it will consistently give solid performance and low saturation voltages.