I want worksheet referenced cell to link to its own worksheet

I am trying to set up a budget with several workbooks.

first workbook is budgeted months to date
if I only have january figures then formula reads

=sum(jan+feb+march)-(feb+march)

Budget is for 10 departments (workbooks).

I want to have a "constant" workbook where I can put in master formulas affecting all 10 other workbooks so cell reference for this workbook could be that formula (above), but it automatically refers to its own cells in constant workbook so all values are zero instead of refering back to department workbook monthly figures.

Anyway, I want to make formula changes in only one "master" workbook and have it affect numbers in 10 others

help