Am I too or not conservative enough for buy and hold estimates?

[Alberto De jesus]

Hello,

I'm attempting to calculate monthly cash flow on properties in low income areas in Massachusetts. I am struggling if I am putting the correct amount of money aside for monthly maintenance and big ticket repairs. Must houses I will buy are early 1900s. 

I am putting aside 10% of rent for monthly repairs. I also am putting 10% for future big ticket repairs, like a new roof.

Do those numbers seem right?