Function in another workbook
 

I have created my own function in VBA called Testdatesv in what I have
called my master workbook. But I end up wanting to use the function
frequently in a file that VBA opens up (via a user open prompt). So
when this file is open I want to call the function from the Master
Workbook. I can get it to work as shown below:

Range(Cells(1, 35), Cells(lastrow, 35)).FormulaR1C1 = "=IF(RC[1]=""Not
in ANSOS"",""NA"",'Position Control-Master Workbook-with
ONESTAFF.xls'!Testdates(RC[1],RC[3]))"


But the problem is that then I cant change the save name of the Master
Workbook. So somehow I want to have a variable name used for the
reference

So:

master= Application.ActiveWorkbook.Name

will get me Position Control-Master Workbook-with ONESTAFF.xls,
but how do I reference this in the call to use the function???