The hours are the hard part
Five days a week is not forty hours of tattooing. Setting up, cleaning down, consultations, drawing, messages and the books all come out of the week, and none of them is paid by the hour. Put in only the hours with the machine running. If you are not sure, count last month's sittings from your diary.
What to do with the number
The hourly figure is a floor, not a price. It is what you need just to cover the month you planned; what you charge above it depends on your experience, your style and how fast your books fill. If your price is under it, being busier will not fix it: every hour you work loses a little more.
Price by the piece from it, too. Estimate the hours honestly, including the drawing, and multiply. A day rate at your floor tells you what a full day has to bring in, and an hour at your floor is the least a small piece can cost, since the setup, the stencil and the clean-down take that long whatever the size.
Tax comes out of your pay
The pay you put in is before tax. The set-aside figure is the same rough share Chairtime's accounting starts from; it ignores your tax-free allowance, so it errs on the high side. Our guide to Self Assessment for tattoo artists has the dates and what you can claim.
For hourly, day rate and by-the-piece pricing, drawing time and quoting from a DM, read how to price custom tattoo work.