The goal for how many people should be able to make it through the insurance exchange is an internal target that administration officials have not made public. It acknowledges that as many as one in five Americans who try to use the Web site to buy insurance will be unable to do so.

The measure is the first concrete performance standard in the 31/2 years since the government began to design the health exchange, and was defined by a group of federal officials and technical experts in late October. It is now guiding the work of hundreds of government employees and contractors racing to try to repair the balky Web site by the administration’s Nov. 30 deadline…

The goal is that 80 percent of people going to HealthCare.gov should manage to enroll electronically — but that means that many others, perhaps tens of thousands, will not succeed. It puts more pressure on the administration to fix technical problems that have made it difficult for people to sign up for coverage by other routes, including federally sponsored call centers and the insurers themselves.