Rather than pay quarterly estimated federal tax payments I have it withheld from my RMD and this year I will use 50%. If I make a large QCD will the IRS withhold 50% of my original RMD or 50% of the RMD I will receive after the QCD?
The IRS doesn’t do the withholding; that’s done by your IRA custodian. You can designate the amount to be withheld.
If you have other income with no withholding (interest, dividends, etc) you can even withhold more from your RMD to cover those tax obligations since you are not filing quarterly.