How do you actually set the reserve line on a 12-unit? Per unit or percent of rent

I've owned land long enough that the holding cost is a tax bill and a mower, so budgeting for a building with roofs and boilers in it is new territory for me. I'm looking at a 12-unit, 1970s brick, collecting about 10,400 a month.

Every model I've been handed does reserves one of two ways. Some put a flat number per unit per year, 300 or 400 a door, on the theory that a roof costs what a roof costs regardless of what the rent is. Others take a percentage of collected income, 4 or 5 percent, on the theory that a building commanding higher rent has more finish and more systems to replace.

The per unit method is honest about the fact that a boiler doesn't care about your rent roll. The percentage method scales with the building and is what most brokers seem to show, which may only mean it produces a smaller number on a marketing sheet.

There's a third camp that says reserves are a fiction on a 12-unit, you fund a real capital account at closing and refill it from cash flow when you draw it down. That one appeals to me because it's actual money in an actual account rather than a line item.

How are you doing it on buildings this size?