Payments upon contracts shall be made as the trustees prescribe upon estimates made and approved by the trustees, but progress payments shall not be made in excess of 95 percent of the percentage of actual work completed plus a like percentage of the value of material delivered on the ground or stored subject to or under the control of the state, and unused. The trustees shall withhold not less than 5 percent of the contract price until final completion and acceptance of the project. The Controller shall draw warrants upon estimates so made and approved by the trustees and the Treasurer shall pay them.