As Beacon Hill lawmakers consider state pension reforms that would embrace a new formula for calculating retirement benefits for public employees, Winchester is taking a lead and setting the stage for a new standard with the contract town leaders approved Tuesday for their incoming town manager. The contract, signed by the Board of Selectmen and Malden Mayor Richard C. Howard, who will take the helm in Winchester on Jan. 3, calls for Howard’s pension to be calculated on the average of his last 10 years of compensation. Currently, pensions for retired public employees are typically calculated based on the worker’s top three years of earnings. Howard will earn $150,000 a year in Winchester.