20 cos u want it to be precise as possible and thats how many volts it will read up to at that setting. for instance if u set it at 600 then you would hardly notice 12v beacause its so small in comparison to 600v
Yup. You want to set it to 20V. A higher setting would give you less precision. Less than the voltage measured (~12V in this case) would give you an overload.