Perhaps one way to try ..
Click Insert > Name > Define
Enter in "Names in workbook:" box: GST
Enter in "Refers to:" box: =10%
Click OK
The above creates a named "constant": GST
that can then be used in formulas in any sheet in the book
For example, in say, Sheet1,
if you have the total for cols A & B in say, col C
i.e. in C2: =SUM(A2:B2), copied down
just change it in C2 to: =SUM(A2:B2)*(1+GST)
and copy down
This will compute the original total amount plus GST
And to update any future changes in the GST percent to apply everywhere
within the book, we just need to use the same route,
via: Insert > Name > Define
and change it there
