Build your own panel meter

The C++ software section responsible for calibrating the device is quite self explanatory.

Adjust from software the values of the resistors you populated on your voltage divider, then measure the MCU voltage and replace this voltage for the ADC into the formula. 

For calibrating the current reading, keep in mind that you can easily have some resistance variations on the shunt and this could be because of your solderings and because of the pcb design. So after you made sure you got the OpAmp resistors correctly declared and the ADC voltage, adjust the measurement from the shunt value. This is very sensitive and a simple change from 0.25 to 0.259 can mean almost 4%. It helps if you use a good calibrated multimeter that you might have around the shop to gain the actual value of the measured current. And since we're talking about multimeters and the accuracy of a good multimeter for calibrating this thing, I will take the liberty of attaching this review I made some time ago:

The ADC returns an integer value and making an average between the last 8 or ten acquired values, can return in the end a float value and the result can be that you can display an accurate value on the display and you can even do so adding some decimals. This comes really handy with displaying some mA.

I added the C code into the Files area. Stay tuned for more details and instructions on how to calibrate the meter according to your resistor values and MCU voltage.

Check the Files area to see and download the schematic and layout files for the panel meter PCB

This is actually the second iteration of the PCB. At first I used one of the PCB's that I had from the Scorpion 3.0 project. I trimmed the extremities of the PCB and used only the middle part with the MCU, OpAmp shunts and regulator. I wrote the first version of software using this refurbished PCB and then I redesigned the entire thing.