My primary goal is to catch anything in advance of someone's lease end date, mitigate ongoing issues, and anticipate future capital expenditures. 

But, you never know.  10% of gross rent for repairs and capital expenditures (capex). 

I purchased this c. 1875 building in September from a slumlord for $520,000 ($43K/unit) and began what I knew would be a serious overhaul to double rents from $700/unit/month to the going market Class B+ apartment rate of $1300/unit/month. 5 months and $250K of rehab expenditures later, the building is completely re-wired and the 12 renovated units are tenanted at rents meeting my projections.

If I could see a 200-300 dollar increase in monthly rent I would go for it but that would mean expensive upgrades to bathrooms and kitchens and I don't think the increased rent would be worth the expenditure-but I have not ruled it out.

Single family homes vs garden apartments vs high rise apartments will all have different ratios and within those classes, will all have different results in different states.It is not possible to get the number you are looking for as an accurate across the board of all investment classes for cap ex or repairs/maintenance.That said, here is a blurb copied form the NAA (National Apartment Association) which puts out data on operating and cap ex expenses for apartments of garden style and high rise: “Capital expenditures, which can include anything from concrete and masonry work to amenity upgrades to extensive rehabs of some units or a clubhouse, has steadily increased since 2010 when measured against GPR.

The tenants rent cover doll vacancies, maintenance, capital expenditures, etc.

We wanted to perform due diligence to see if that type of capital expenditure was worth it.

Of course they have the capital to ride out a few years in the red and absorb any additional capital expenditures before they start seeing returns, but as a long term strategy it's a great wealth multiplier. 

Though probably does not include all the capital expenditures of the last few months - boiler etc. 

It's paying for all your expenses (mortgage, insurance, taxes) and then setting aside an additional percent (usually around 50% of the rent) to cover future maintenance, capital expenditures, vacancy, property management, etc.If you took 50% of your rent for expenses, that would leave you $1,200 to cover the mortgage and insurance.