50 mV is the critical parameter, what the meter reads can be changed, if you say wanted to measure 15 anps then you need a 15 amp shunt that give 50mV @15 amps.
Or say 50 amps then you need a shunt that gives 50 mV @ 50 amps, the scaling on the meter can always be changed it's only a visual thing.
Choosing a shunt that is way over range ie a 150 amp shunt when you only need to measure 15 amps will not be very accurate at the lower end of the scale.
allan down under