How do YOU calculate R&M and CapEX

I Would like people's thoughts on how YOU calculate cash flow on a rental property, specifically expenses for repairs & maintenance and CapEx.

The way I do it is I have a cost I've developed that I use for monthly R&M and ALSO I use a property specific CapEx calculation using a spreadsheet that takes into consideration of each CapEx item's cost new, how old it currently is and how much per month you would need to set aside to replace that item. For example: If you have a roof that is 15 years old and has an effective life of 30 years. It cost $4,000 to replace that roof new, I know I would need to set aside $22.22 per month ($4,000/(15 years*12months) or $4,000/180) to recoup the cost to replace that roof when it's effective life is up.

What method do you use and what works for you?