Regarding the sums in your post:
If 15% = £7000, divide by 15 to get 1% and then multiply by 100 to get 100%. That yields a figure of £46,700 (to the nearest hundred pounds) as the current annual salary used in those calculations.
A key bit from TCL's quote is the "before expenses" bit. Barristers have to pay fees to their chambers. Further, many barristers live in or around London (because that's where their chambers are based) but have to travel to Crown Courts across the country. So, for example, a barrister representing a client in Norwich Crown Court will have to pay a peak-time return rail fare to get there before 10am, whereas a barrister who has to go to Newcastle Crown Court will need to pay the costs of a return rail fare and a night's accommodation. So barristers' expenses can add up very quickly.
The median figure for a barrister's pay two years after being called to the Bar is just £18,800 after their expenses have been deducted. That mean's that a half of all barristers with that length of service are earning LESS than £18,800. (Source:
https://www.bbc.co.uk/news/uk-62629776 )
That's after three years at university (where exceptional A-level grades are required to get onto a Law course in the first place), another year of study for the Bar Professional Training Course, a year of pupillage in chambers and then those first two years of service.