Understanding the 50% rule

[Ryan Standage]

I have been reading up on the 50% rule, and i am trying to understand it fully. My previous method of calculating expenses only adds up to a mere 35%

my old way includes

Property tax x 12
Mortgage insurance x 12
Repairs & maintenance @ 2.5 % of property value
HOA x 12
Home owners insurance x 12

the sum of all these number / true annual rents

so what am i missing in the in my old way that doesnt add up to 45% plus or minus 2%?

do my repairs need to go up?