one possible solution might be using chart inter record functions, like above().
If you have a chart with dimension month of year and you get the revenue for a month with an expression like
=sum( MonthRevenueField )
then your expression could look like
= sum( SomeAmountField) / ( sum( MonthRevenueField ) + above(sum( MonthRevenueField)) / 60 )
(Not sure about the correct use of brackets around the division here, I tried to copy from your above post).
Above function will evaluate the expression inside in the context of the dimension one row above (in a line chart with dimension month of year, this should be the previous month).
For the very first month of year, you won't get a value for the previous month this way, so this might be a problem.