Global calculation could be interpreted as different things. So I am not sure if I interpret your question correctly - I'll give it a go...
Is it calculations that are not just in one application? Some shared calculation that you need to do in multiple QlikView applications? Then you can store the expressions in a database of your choice, text-file or even an Excel workbook/worksheet file. You will have to read in the expressions via the load script and store them in variables that can be used in your QlikView chart objects and UI.
Is it calculations that are global within just the same application? You can also here use variables to do the calculation or just store the expression for a calculation and then use these calculations/expressions (stored in variables) to use them many places in the same application.
Expressions are always calculated within some context and if the context change from one chart to another the dimensions used might be different. This will naturally determine exactly what will be calculated. But as long as the expression is built and used with that in mind it can be very useful.
