Rules of the commission shall govern compromise payments where the prescribed schedules are not applicable and which, in its discretion, may be made in cases where it is not possible to determine the exact extent of disability, as for example in certain injuries to the back or head. The commission shall also have full authority to adjudicate the disposition of death claims. Commutation and lump sum settlement payments shall be governed by rules of the commission, and shall not be made except when determined to be in the best interest of the injured worker or his dependents, the commission having final authority in such questions.