Pay yourself too much salary and you waste personal allowance against National Insurance. Take too much dividend and you push yourself into higher-rate dividend tax. The calculator tests every salary from £0 to £60,000 in £10 steps and returns the split that leaves the most cash in your pocket after all taxes.
Optimal split found by testing salaries from £0 to £60k in £10 steps. Assumes no other income, no student loans, no pension, standard personal allowance.