Lifetime Income Illustrations - What is the formula?

I have been searching and searching the regs and IFR for this to no avail. I understand what data pieces need to be used to generate the lifetime income illustration but what is the formula to generate the monthly payments. I imagine it must be so easy that no one else is asking because I do not see it. If I get a question from a client on how the monthly payment was calculated I need to be able to provide that. I've reviewed the standard annuity formulas but those do not seem to be what I am looking for because they require you to know what the monthly payment will be. 

The formula appears to be a very simple calculation of the account balance divided by the annuity purchase rate.  Two of them, in fact, because the disclosure is required for both single life and 100% joint and survivor (assuming spouse the same age as the participant, I think).  You calculate the annuity purchase rate using a published interest rate (I think it was 1.2% last time I looked) and the 417(e) mortality applicable to the date of the calculation.  

I have a recent post where I displayed how my program can be used to do the calculation.  However, it shouldn't be necessary to purchase a separate program to do this as every admininstration system will have a method for doing the calculation.

I believe C.B. Zeller has published a free excel worksheet that can do the calculations: 

Thank you so much Mike, this helped a lot!