Help estimating CapEx: Analysis shows $800 for $250,000 4-plex?

[Taylor Thompson]

Hi, I'm trying to decide whether to make my first offer on a 4-unit, and my cashflow analysis depends entirely on what I budget for monthly CapEx. My estimate shows around $800 in total CapEx, which amounts to 20-30% of monthly rent, depending on what renovations I make. This seems higher than the typical 5-15% "rule of thumb" people use, so I'd appreciate advice from someone more experienced to see if my attempt to be conservative is overestimating CapEx.

Here are some details on the property: 

• New England building, 3,000 square feet, built in early 1900s
•  4-Units: two 3BR/1B, to 1BR/1BA
• Property has had recent roof and heater replacement, but has had same tenants and little interior work done in 10 years; Class C property in Class B area 
• Total market rent today of $3300 per month; $4,100 after $80,000 in renovation
• I assume 10% repair cost before renovations, 5% after renovations 

Here's my CapEx Calculation (with Cashflow analysis on second tab). Can anyone spot any beginner mistakes that I'm making here?