Percentage of completion method required for construction and service contracts; short-duration and indeterminate services use alternate methods. Income from construction contracts and contracts for providing services is determined by the percentage of completion method, with short-duration service contracts accounted under the project completion method and contracts comprising indeterminate acts accounted under the straight line method. For these purposes, contract revenue includes retention money, and contract costs are not to be reduced by incidental income such as interest, dividends or capital gains.