Retirement Calculator (of sorts) Needed

[Nathan S.]

I currently have three rental units (SFH, town home) and my goal was a total of six.  However, I have decided I wanted to move up my timeline for retirement to age 55 (I'm 38 now) with my original math telling me I could have them paid off between age 59-61.  

Does anyone have a spreadsheet or formula to determine how many additional units would be needed to increase cash flow to pay off the first initial rentals on a more accelerated time table?  $300-$350 of cash flow per unit is my minimum accepted in order for me to justify the investment.

The idea would be to have the first six paid off by age 55, leaving me at a comfortable retirement income.  Thanks in advance for any insight!