The courts generally endorse the insurance firms' deduced day rate, whatever that is these days. Alternatively you can present a set of audited accounts.
I made up a figure when I had to leave the MKIII in for repairs to damage cause by a TP. On that occasion I got a cheque from the insurer within a couple of days so I assume whatever I asked for, while unsubstantiated, was in the ball park of their deduced rate.