Contractors split indirect costs two ways. Job indirect costs belong to one project, such as a superintendent or temporary power. Company indirect costs are office overhead that every job shares.
Example: a mechanical contractor carries a project manager, a site trailer and safety costs as job indirect costs.
Estimate job indirect costs item by item, using the schedule length. Recover company overhead through your markup. Be consistent about which bucket each cost goes in. A longer schedule raises indirect costs even when the quantities stay the same.