you can measure your GPH by timing how long it takes to fill a container. Every setup is different based on height and restrictions from different media/cleanliness.
Also, you can't base your filtration on turnover rate or GPH, you need to take into consideration the type of media and filtration method that you're using.
For example, and empty AC110 turns over a lot more water than one full of biomax, but does almost nothing for filtration (extreme example, I know)