CRANSTON — School officials have revised their estimate of the cost of a tentative contract agreement with the teachers, lowering the projected net savings over three years by $400,000, to $1.8 million.

Joseph A. Balducci, the district’s chief financial officer, said Wednesday that the original estimate was based on some erroneous assumptions — on both the plus and minus sides.

In one change, Balducci said the district is no longer counting $1.5 million in savings from new guidelines for teacher planning time because the contract would only help the district avoid future costs, not reduce current costs.

The new savings estimate also reflects the cost of restoring program supervisors. The supervisors were eliminated for 2008-09 but will be back next year, with more classroom time than they had in the past. The added classroom time will help to reduce the overall teaching staff, but the $2.1-million cost of restoring the supervisory positions was not included in the original net-savings figure, Balducci said.

Finally, the new savings figure does not include the cost of state-mandated step increases, because the district would face that cost — $2.9 million — with or without the new contract, Balducci said.

The contract, approved by the 970-member Cranston Teachers Alliance last week, is scheduled for a School Committee ratification vote on Tuesday. The board had been expected to vote Monday night but held off because the new contract had not been posted on the district Web site, as required by city ordinance, said board Chairman Michael A. Traficante.

The contract would give teachers a 1 percent pay increase, at the top step only, during the coming fiscal year and 2.25 percent increases at all steps in the following two years, in addition to annual step raises.

It also would end a legal battle between the district and the union by bringing back the program supervisors, allow teachers to enroll domestic partners on their health insurance plans and allow teachers to take home the equivalent of 100 percent of their pay when injured on the job, by supplementing the 60 percent the district pays with unused sick time.

The biggest savings for taxpayers — $5.4 million over three years — comes from higher employee contributions for health insurance. Teachers, now paying 5 percent of their premiums, will pay 15 percent during the next two years and 17 percent during 2011-12.

The contract also saves money through schedule changes that give middle and high school department chairs more classroom time, and health-plan changes that will charge employees more for some prescriptions and medical visits. Those savings are offset by rising costs in areas such as salaries and pensions, leaving the projected three-year net savings of $1.8 million, according to the latest analysis.

redgar@projo.com