You're just a bit too much a programmer, and a tad too sharp an entrepreneur.
Become somewhat more a manager and factor in some 'fat' -- both in the invoices you charge your customers, and in the time you allocate your contractors. Remember, crap happens. If it doesn't, and your contractors manage to deliver bug free code ahead of time, give them a 'performance' or 'quality' bonus (i.e. payment for the full time allocated). Do not chase profit maximisation like big shops do. They have adequate manpower buffers to beat you at your own game.
Alternatively, hire someone skilled --- but don't be cheap. Remember, if you chase cost reduction like the small shops, your employee obtains the hunger (he already has the skills) to beat you at your own game.