Well according to me.. if you have 4 zones and those make area to 100% then you make take percentage of percentage to combine GAM rates:
For instance:
Zone 1 is 15% of total area and GAM rate is 12% so this means actually GAM rate is 1.8% (15%*12%) and if you take same for all remaining zones, then you might get what you need.
This is just a way to do it.. may be you find some technical way of doing it.