I would advise you to do all the calculations in one sheet - from what
you have said this may continue beyond 12 months, so you will
accumulate more and more sheets otherwise.

You may then have two or three summary sheets giving you (or your boss)
different views of the data. You can obtain summaries of the income for
all months (or selected months if you prefer) by referencing the data
in the main sheet - you need to have a clearer idea of how you want the
data to look. If you follow my earlier suggestion and have the totals
in the top row of the main sheet, then you would have something like:

=main!R1
=main!S1
=main!T1

to bring the data from cells R1, S1, T1 etc into another sheet. You
will probably also want to bring the month from the header row below
it.

Hope this helps.

Pete