There is no sorer subject than San Francisco’s housing crisis, where rents soar past $3,000 per month and an average home goes for over $1 million. But beware the solution contained in Prop. C, a measure that obliges builders to set aside 25 percent of new units at below-market levels.

The idea is appealingly simple. As developers rush to cash in on rising prices, there should be ample money to save a larger slice at discount prices for residents being priced out. The city now has a 12 percent requirement that should be boosted in a booming market.

But this higher number doesn’t rest on any justification. The prime sponsors, supervisors Jane Kim and Aaron Peskin, say it originates from past deals that increased so-called inclusionary housing in developments sponsored by the Giants and others on Treasure Island and Hunters Point. But these projects come with major concessions such as cheap land and other subsidies that made the numbers work for sponsors.

Prop. C’s boosters offer a promise. A city controller study will consider the 25 percent figure to see if it will choke off new building as some housing experts believe. If a lower number is warranted, sponsors promise to bring it down to sustainable levels. But that’s only a pledge and one that may be hard to honor in the super-charged atmosphere of housing politics.

The measure comes with City Hall’s big blessing, with all 11 supervisors and Mayor Ed Lee in favor. Prop. C gets at least one thing right: It removes the subsidized housing figure of 12 percent from the city charter and leaves it to lawmakers to set as conditions warrant.

Builders and developers, who might be expected to oppose any exaction on their trade, are going along with Prop. C. Why? Because a long line of projects awaiting approval will be exempted after late-inning negotiations. This deal-making bought off the measure’s would-be opponents.

The measure is a flawed way to deal with an important subject. Subsidized housing should be encouraged, but not with arbitrary numbers and no serious study.

Vote no on Prop. C.