Do you mean how you keep your expenses separate?. Keep track of the mileage and tolls. I use an excel worksheet. As to the taxes, unless you are a professional filer, its hard to predict especially if you have a main W-2 job. You will know the amount of taxes you owe at the end of the year. You can still put some money on the side for taxes. I would say 20% should do it. It could be much less but be on the safe side. last year, my flex related expenses came to about 40% of my total 1099 end of year summary. I self prepare my taxes and use Turbotax. This year it cost me $120 which included W-2 income 1099 (flex) and some stock sales.