A proposal to require new oil and gas wells to be at least 2,500 feet from homes and schools in Colorado would leave 90 percent of the state off-limits to future drilling, regulators said Friday. The Colorado Oil and Gas Conservation Commission released a report on the impact of a proposed constitutional amendment that backers hope to put on the November ballot. The report said that in the state’s top five oil and gas counties, 95 percent of the land area would be off-limits to new wells and other energy facilities.