How this is calculated
Your take-home target is grossed up for tax, then business costs are added, giving the revenue you need to invoice. That total is divided by your billable hours for the year.
The number it gives is a floor, not a price. It's what you must average to hit your target — charge below it and the year doesn't work no matter how busy you are. What you actually charge should be higher, because some hours go unpaid and some clients disappear.
The mistake this exists to fix
Beginners divide a salary by 2,000 hours and call it an hourly rate. That ignores tax, costs, holidays, sick days and the fact that perhaps half your week isn't billable at all. The result is a rate that looks fine and quietly loses money.