I generally take what my normal overhead a profit would be on an employees wage and add it to the prevailing wage rate to remain somewhat competitive, sometimes it works, sometimes it doesn't.
You need to keep in mind on these jobs as well you WILL be audited for payroll, so expect to have someone on site who does nothing but watch the guys and track there every move and record it as that is how you will be paying and in your office the bookkeeper should be compensated at the hire wage while doing and record keeping of the project as well, including books, payroll, audits, etc.
This will be an eye opener for you.