Don't worry too much about percentages, you don't need to do the sums yourself! When you fill in an online tax return it's calculated for you, you just have to put your data in Just use the percentages as a guide to what you need to put aside for paying your tax bill.
NI is paid separately by direct debit each month, so don't worry about this.
Exactly, Roxy - you're overthinking, that's all! Although I suppose it's possible that we're all Mensa geniuses too
It's a good idea to put about 30% of your earnings aside into a savings account just in case you end up having to make some tax payments at the end of the financial year. And you must pay your NI contributions weekly or monthly by direct debit but they are only a couple of pounds. If it gets to the end of the tax year and after filling out the form ("I earned x on this day, x on that day but spent x on condoms too" etc etc is what I imagine it will be like - I haven't worked a full tax year yet either so I've got the same experience to look forward to!) it does your calculations for you and says, "Oh, you only earned ?6,003" then you will be able to cackle to yourself and consider your tax savings to be your own money again!
On the other hand, if you don't want to work that many hours as an escort and were only thinking of doing it for a few months and never again, it might be easier to try perhaps a bar job for the summer? I don't mean that rudely at all - just that then obviously you need not do a jot of tax form nonsense and can get on with earning loads of tips! Plus, it would be a more social job, not to mention one you can actually tell your friends + family about. I dunno. It's entirely up to you and I don't mean to suggest that you can't do escorting or anything! But honestly, the tax bit is the easiest bit cos after this you've got to either sort out a trustworthy & reliable agency or launch as indie with a security process set up and possibly an incall location and/or travel arrangements, pictures, website, all that. Of course I know you can do all this easily - but for 3 months of working, maybe it would be honestly quicker to earn money for uni in one of the other fun jobs? I know I ended up waiting til after my degree to start escorting - I had WAY too much else to be thinking about before that!