CaPex Budgeting Costs

[Jamall Lynch]

Top of the afternoon BP family,

I'm a newbie who recently came across a formula on how to budget for CAPEX. It says that I should take the cost of the repair and divide it by how many years it will last and divide that number by 12 months to get the dollar amount that I should be tucking away every month for that expense.
For Example: Let’s say it’s a $20,000 roof that will last 10 years. 

20000 ÷ 10 ÷ 12 = 166

This formula states that I should be setting aside $166 per month for this expense. 

I've also heard a few investors say that it's a good rule of thumb to set aside 10% of my gross income every month for CAPEX. Which metric is best to use when accounting for CAPEX? And what are some additional CAPEX items that I should be factoring in besides the roof and HVAC? I'd greatly appreciate your input.