25% of the withholding will be credited to each of the tax quarters, so you might want to have withheld from your RMD twice what you should have paid for your Q2 estimated tax. The 25% of the withholding credited to Q1 and the 25% credited to Q2 will then cover what you should have paid in estimated tax for Q2. You can then reduce your Q3 estimated tax payment (if not already paid) and your Q4 estimated tax payment to account for the 25% of the tax withholding that will be credited to each of those quarters.
