To calculate your Required Minimum Distribution (RMD):
For example, if your account balance is $100,000 and your distribution period is 25.6, your RMD would be 100,000/25.6 =3,906.25
Alternatively, a simpler way may to be to o use online calculators like this one from Calculator.net or Investor.gov to help with the calculation
What do you have a 1099R for? If it is for a pension, Say all of it is the RMD. If you need to take the RMD it will ask you if it is the RMD. Say yes. Anything your pension pays you is considered to be the RMD. Traditional pensions automatically fulfill the rules of an RMD. So for the RMD question put in the same amount as box 1.
If it is for a IRA or 401K you can ask the plan. Did they tell you how much to take at the beginning of the year? Or did you decide how much to take out?