The easiest way that will really hit you is, are you turning away business because you can't handle the load?

If that is a yes, then you have to say, how much am I willing to pay a person to do the extra work?

Lastly, the business you are turning away, is it enough for you to pay a person, and still make money or at least break even.

The last one is the one that should really be the decider. If you can pay the person the salary you set in the second question, and still make money after expenses, then yes, you should hire someone. The trickier part is if you only break even.

The reason it is trickier is, maybe based on your current numbers of turned away business you break even, but wit that new employee, you could see an increase in business due to word of mouth and that sort of thing.

The only time you say no to hiring is when it would not make you money, or break even after you hire.