This is an important entry. If you don't know the answer and the form issuer does not give you that information, you need to call them and ask. Fiduciaries issuing non CSA-1099-Rs usually will put a note along the bottom of the form or in an attached letter specifying the amount. Your amount may have been detailed on your pension check summary or stub.
You really don't want to guess and, as far as I know, it's not a calculation you can make yourself if there's no information on your check stub or 1099, it's a very specific amount that the issuer would know.
