You will have to do some interpolation. Your digital meter should have a high input impedance and be a negligible load. You could use 2.5 as full scale but you would need a 12500 ohm resistor which is not common. 15000 is very common and you would use 3 volts as full scale. The scale on your digital meter should make no difference as long as it is above 3 that is 10 or 20 vac. For instance on the 300 amp scale the meter would read 3 volts or 200 amps will read 2 volts.
On the red 12 amp scale the multiplier is 4 and 3 volts = 12 amps of 2 volts =8 amps and so on
If I was doing it, I would use a 20K potentiometer and place across the meter leads and calibrate the reading to match the reading of a known good meter.