CapEx - Am I doing this right?

[Josh Buss]

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?