Yeah, using map is still an excellent idea.
Let's say that you want to use map. You input a regulated 12 volts. You test with a multimeter and sure enough it's 12V exactly. So you hook it up to the voltage divider and hook the voltage divider up to the arduino. Then you output what readings you are getting. Let's say that you let it run for 5 seconds and there is one reading a second. The readings are:
923
921
923
922
922
So you average them up and get pretty close to 922. Since you now know both the exact input voltage and the averaged reading you can plug them into map:
voltage = map(analogRead(Vin), 0, 922, 0, 1200)
This gives you a hundredths of a volt reading that is calibrated to the hardware at hand.