The Fly-E3-Pro natively supports input voltage monitoring.
This allows for resume on power loss to be used. See here for more information.
On testing, running 24v, we’ve found that there is enough power to write the resume information to the SD card but there isn’t enough power to lift the head away from the bed. Therefore, it would be beneficial to add a large capacitor across the input voltage.
We suggest running a print and monitoring the input voltage, periodically checking M122 for the lowest value (with everything running e.g. fans).
Then use the minimum voltage - 1v in your M911 command. E.g. if your minimum voltage seen during printing was 23.8v, set the S value to 22.8.