What forrest said.
Your income must exceed your outflow.
What do you project for a year?
What are your slow months?
Here is what I do, since I'm a mini-micro company.
I target working 4 days a week, weeks a month. Or 192 days a year.
So if I average a gross $200 per day I will do 38,400 per annum.
Go through your calendar and cross off all improtant holidays that you definately will not work. Wich week days will be your short or off days, do you forsee yourself honoring Shabott and knocking off friday afternoons?
How many vacations do you want to take durring the year, and when?
When do you usualy have raindays?
After doing all this you will probably find your self near a minimum work year of 200 days too (sat/sun add up to 104 days alone).
Now find out what you need for a minimum ave daily gross to hit your target minimum budget. How many jobs will you need to be able to meet this goal?
As otheres have said, unless you are doing more then just tree work, then 30-40 client is a bit low. even if you average 2 days of work per year from each client, that will leave you with around 120 days to make up with single shot customers. You will always have to do this, to some extent, cant rely on the customers being your gravey jobs to pad the income.