Federal Tax withholding for Reservists

I am wondering if anyone has advice/best practices for Reservists to deal with Federal Tax withholding. Mypay assumes that I am either an hourly employee or a salaried employee or gives the option of a 'flat rate.' Usually, I don't withhold anything and just wait until I'm told what I owe at the end of the year- but this also seems pretty dumb. The system was not like this prior to the 2020 changes (back when S1 submitted all your documentation). I'm not here to whine about it- just see what any other Reservists do so I can find a better solution than I currently have.

Thanks for any input or for sharing how you have been successful at estimating and inputting your Reserve Federal withholding balance.