My wife an I just put an offer in on our first property, a singe family in Oklahoma City. To get an accurate (probably high) estimate of the per month capex costs over the twenty year period that we plan to hold the property I went through everything in the house (using @J Scott book) that will likely need to be replaced one or more times prior to reselling the home. I then divided that number by 240 months and used that to create the percentage that I entered in to the BP rental analysis calculator. 

Am I missing anything?

Well, without knowing what you you planned to replace, it's impossible to know if you missed anything but good for you for being thoughtful and working the numbers instead of just throwing a happy estimate at it.

Do you have the numbers you came up with? I think that would help us evaluate it.

One thing you're not taking into consideration is increasing prices over 20 years.  What costs $100 now might cost $115 in 5 years, considering a 3% price increase year over year.

@Deborah Burian thanks for the input!

These are the cap ex costs that I came up with for a brick 800 square foot 2/1 SFR that I put an offer on this week. The expenses are over 20 years and are adjusted for inflation in the rental properties calculator.

• Roof- current is two years old, figure one replacement- $5,000
• Gutters- one replacement- $480
• Soffit/ Fascia- one replacement- $480
• Exterior paint- $900
• Water Heater- $500
• HVAC- $2000
• Doors- $300
• Windows- $1200
• Interior Paint- $4,050
• Cabinets- $1,300
• Counter tops- $400
• Flooring- $4,000
• Fridges- $800
• Ranges- $800
• Dishwasher- $500
• Washer/Dryer- $1000