Thanks Master Caution, that helps, and as bug squash said, about 15% seems to be the figure.
Not sure I understand why you would have to plan on 30% for the first year? If you pay one year up front then you have a year to save for the 2nd tax year no? In which case after the initial 1st lump payment you have to put aside 15% a month?
Is housing allowance taxable too?