How are taxes calculated on Mandatory Disbursements?

At the end of the tax year 2025 you will get a 1099R for it in Jan 2026.  It will add to your total income and be taxed as ordinary income at your tax rate.   It’s not taxed as capital gains.  

Or do you mean how much tax withholding will be taken out?  You can usually tell the plan what percentage to take out or they might take a default 10% for federal.  

There is not a "special" tax rate for an RMD.    Your distributions from your retirement account are taxed like your other income.   You will receive a 1099R at tax time which you will enter on your tax return.   Your RMD might cause your Social Security to become taxable or might cause more of your SS to become taxable, since the taxability of your SS is determined by the other income you are receiving.

Oh why do you say March 2025?  Do you need to take your first RMD for the 2024 tax year?  You need to take the RMD by Dec 31 each year.  But you can delay taking the first RMD by April 1 of the next year.  But that means you will have to take 2 RMD in 2025.  So don’t forget to take the second RMD.  The first RMD in March 2025 will be based on your 2023 ending value and the second RMD is based on your 2024 ending balance.  Or you can take the first RMD this year in 2024 instead of taking 2 in 2025.