A good starting point is to calculate your annual fixed costs. eg. car lease, insurances, premises, telephones etc. This will give you a base figure for your fixed costs that must be recovered before you can actually earn anything for yourself.
Add to this a typical average salary (in the UK I use 24K as my usual base figure).
Put these figures into a spread sheet and then divide by 200. I use 200 as it is roughly based on 48 working weeks per annum X 4 paid days per week (this leaves time for non-paid work ie. bookwork, promotions etc) and it is an easy figure to remember 
This will give you a minimum day rate.
At this point you now have your base monthly fixed costs which you can now detail and layout on a forecast which properly reflects when these costs are due. You also have a minimum required income per month which you need to work towards achieving.
Now you can do your business forecast based on how many paid days per month you expect to work and from this calculate a best guess of variable costs associated with orders and variable income from these orders.
Once the business forecast is done you can now do a cash flow forecast which identifies how much you expect to go out in any month and how much to come in and should show the point at which you move to positive cash flow.
You now go back and review it to ensure you have covered all expected costs and incomes for the period of the forecast.
Once you have done this you realise that the quickest way to make a small fortune in photography is to start off with a large fortune
