A mini-controversy has broken out over the way the Environmental Protection Agency calculates the legal level of lead in gasoline. And top EPA officials are "delighted," sources say, because it gives the Reagan appointees a chance to be tougher than the Carter administration on an issue that they don't consider major.

The issue broke out Tuesday, when the Center for Auto Safety released an agency letter that said EPA allows refiners to sell gasoline with lead levels of up to 0.549 grams per gallon, when the legal limit is 0.5 grams. The group charged that this would allow refiners to increase the amount of lead in gasline by 10 percent.

But Richard G. Kozlowski, a long-time EPA enforcement official who directs the program, said the standard has been calculated that way since it took effect in October, 1980, and has never been a secret. He added that EPA uses that method when it certifies cars for emissions of hydrocarbons and nitrogen oxides, a program that has been in place since 1968.

Clarence Ditlow, executive director of the auto safety group, said yesterday that he still thinks the Reagan administration implemented the policy "secretly" to help refiners raise the lead level of gasoline. The agency has been considering relaxing the standard.

Meanwhile, EPA spokesman Byron Nelson announced yesterday that there is "an active study of this issue at this time." There "is some concern here," he said, noting that the policy was developed during the Carter administration.

Nelson said a final decision would be announced when questions on overall lead standards are made "sometime this summer." He said the issue of rounding off was raised several weeks ago by an environmental group. But he said EPA Administrator Anne Gorsuch's top aides, who do not follow the program on a day-to-day basis, were unaware of the situation until 10 days ago.

Kozlowski said that, when the agency began enforcing the standard, refiners asked for guidance on what level would be considered permissible. Based on a rounding method established by the American Society for Testing and Materials and a legal opinion by the mobile source enforcement office, EPA set the enforcement level at 0.549, according to Kozlowski.

"It's consistent with what the agency is doing with other standards. It's an accepted scientific practice," he said.