In case of temporary partial disability the weekly loss in wages shall consist of the difference between the average weekly earnings of the injured employee and the weekly amount which the injured employee will probably be able to earn during the disability, to be determined in view of the nature and extent of the injury. In computing such probable earnings, due regard shall be given to the ability of the injured employee to compete in an open labor market. If evidence of exact loss of earnings is lacking, such weekly loss in wages may be computed from the proportionate loss of physical ability or earning power caused by the injury.