T J = (m ´ V F ) + T 0 (1) where T J = junction temperature in °C m = slope (a device-specific parameter) in °C/V V F = forward voltage drop T 0 = intercept (a device-specific parameter) in °C ...

Instead he looks at using the 1.1V internal reference. He’s written some short example code that let’s you measure the incoming line voltage based on that internal reference.

That resulting signal is a square wave, which is an easy target for the Arduino to measure. That timing measured from the square wave can then be used to calculate the inductor’s value.