Lockheed Corp., the nation's eighth largest defense contractor, reached a tentative labor agreement with the union representing about 20,000 workers for the aerospace firm. The tentative contract reached early Sunday still must be ratified by rank and file members of the International Association of Machinists and Aerospace Workers. A nationwide vote was scheduled for March 4. ``We feel it's the best contract that we can negotiate at this time, especially with government audits,'' union negotiator Bob Gregory said Sunday, referring to government monitoring of Lockheed's hourly compensation costs. Tom Goff, spokesman for the Calabasas-based Lockheed, confirmed that the two sides had reached a tentative agreement. A week earlier, union members rejected management's latest offer by a 98 percent margin. The union said Lockheed's earlier offer was inadequate and set up a two-tier wage structure. As part of the tentative agreement, both sides declined to disclose terms of the contract until the rank-and-file votes. The union workers have been operating under an extension of the previous three-year contract that expired last October. The present contract offer will be retroactive to October and runs through March 1, 1993, Gregory said. The union's Local District 727 represents 6,000 workers in Burbank and Palmdale. The contract also covers workers at Lockheed plants in Sunnyvale and Marietta, Ga. Contract talks had resumed Thursday in Florida and concluded early Sunday. Asked about changes in the contract offer's pension plan, Gregory said that it establishes an employee stock ownership plan for union members. Lockheed set up an employee stock-ownership plan for salaried employees last April. The plan, designed to strengthen the company against take-over attempts, gave salaried workers control of 17 percent of the company.