Each project is different and may require a different amount of the overhead.
Usually you would add up indirect costs like indirect materials or indirect labor first.
Then you would compute the overhead allocation rate. For example, if you use indirect labor, you would divide total overhead by the number of the direct hours.
The third step is to apply the overhead by multiplying the rate by the number of the direct labor hours of each project.
Hope I understood your question correctly.
Let me know if you have more questions,
Elizabeth Boals