I use mine on busines about 15/20 % and charge myself an hourly rate which i have calculated to be a reasonable cost of running the thing for the year (all costs maint, hangarage, insurance, fuel etc) divided by the total number of hours estimated usage that year.
For my aircraft works out at £150/hour. My business reimburses me that ammount.Same as if I were using the Train.
I would be happy for tax man or the CAA to look at the arrangements.