HUD shall establish operating cost standards based on the average annual operating cost of comparable housing for the elderly or for persons with disabilities in each field office, and shall adjust the standard annually based on appropriate indices of increases in housing costs such as the Consumer Price Index. The operating cost standards shall be developed based on the number of units. However, under the Section 811 Program and for projects funded under §§ 891.655 through 891.790, the operating cost standard for group homes shall be based on the number of residents. HUD may adjust the operating cost standard applicable to an approved project to reflect such factors as differences in costs based on location within the field office jurisdiction. The operating cost standard will be used to determine the amount of the project assistance initially reserved for a project.