Change text to numeric format
double(GetValue("ASSEMBLY_TOP_LEVEL"))
Change into correct format for calculation (double=decimals)
format(double(GetValue("ASSEMBLY_TOP_LEVEL")),"Length", "mm", 1)
Add all above into calculation formula
format(double(GetValue("ASSEMBLY_TOP_LEVEL")),"Length", "mm", 1)+15000
Another example of the same for part elevation
(double(GetValue("TOP_LEVEL"))-(double(GetValue("BOTTOM_LEVEL"))))*1000