You estimate that you will be able to invest $300 consistently at the end of each month during your working years, which you project will be for 38 years. 

What is the maximum you would be able to withdraw at the start of each month of retirement?  

Assume a retirement period of 25 years and a return of 8% over the entire planning period. ALSO, assume that you already have $20,000 to invest today (and it will also earn the rate of return you are assuming for your monthly investments), AND that you would like your portfolio balance to be $500,000 at the end of your retirement.