I figure at least an extra 15% of my take for 1099 work is required to cover my "self employment" taxes.
But, the great thing is (I think), when you have your own business, you get to deduct all the expenses on a seperate form, and it is not like the $5700/itemized limit requirement for a 1040.
So, you really will need accurate records of all expenses to max out your deductions. And, I would say pay something each quarter to avoid a penalty at the end of year if you might owe $1000 or more.
I am no expert on this though. I use TurboTax myself, but a good accountant can offer a lot of help if you are willing to shell out the cash for them.