So I retired under FERS on 12/31/18. OPM just finalized my case and my first full annuity will be on 5/1/19. I've been receiving partial payments for the last several months and only a fraction of the Federal tax has been withheld. I used the IRS Withholding calculator but the numbers were not right. So I'm wondering if there is another method or site that would allow me to determine what I should withhold for each month going forward. I know OPM withholds at the married filing jointly with three exemptions, but I think that may be too much. Thanks for any info.