In the June election, San Francisco voters unwisely chose to battle the city’s housing crisis by requiring developers to set aside 25 percent of new unit construction for below-market-rate rentals. The initiative, Proposition C, more than doubled San Francisco’s previous affordability requirement. It was criticized by Mayor Ed Lee and developers for being an arbitrary number that would freeze new building.

So Supervisors Aaron Peskin and Jane Kim, who promoted Proposition C, pledged to adjust the percentage based on a feasibility study from the city controller’s office. Now Controller Ben Rosenfield has released a new draft recommendation that spells out the obvious: 25 percent is way too high.

In fact, even 20 percent is too high.

Eighteen percent affordable units is the maximum that new apartment buildings can rent before San Francisco will start seeing a negative impact on new housing, the controller concluded. Beyond that, the value of the land drops — and owners won’t sell.

New condominium buildings can afford 20 percent affordability before new housing will be impeded, the report concluded.

But most San Franciscans are renters, not owners, and the incredible cost of homeownership in this city means that a majority of San Franciscans will remain renters for a long time to come. For their future — and the city’s future — the cost of rent is what matters.

The magic number is 18 percent.

Keeping rents at a manageable level in a popular city means that San Francisco must continue building new housing units. This is a basic lesson of supply-and-demand economics.

This lesson was underlined by recent research from RentCafe, the property management software company. Earlier this month, RentCafe said San Francisco will add almost 9,500 apartment units this year. That’s the highest inventory growth, in terms of volume, that the city has had for the past 10 years. With rental inventory up by 126 percent over 2015’s numbers, renters are seeing less competition at open houses — and landlords, especially at the top end of the market, are tempering their demands for rental increases.

In June, Equity Residential, the country’s largest publicly traded landlord, lowered its revenue growth expectations for this year. One reason it cited was slower growth in San Francisco and the surrounding area.

All of the signs point to increased inventory making a difference for renters — but if San Francisco stops building, things will change quickly.

So the Board of Supervisors needs to heed the city controller’s warning. There is a point at which making demands for below-market-rate housing will discourage building altogether. If the supervisors decide to ignore the numbers, every San Franciscan will suffer.