FlowCounters library

hi turgo,
i want to use your lib.
can you explain how to get the flowconstant?
how can i set

FlowCounters hotWaterFlow(10,7.5)

for the G3/4 flow sensor from seedstudio (FS300A):
http://www.seeedstudio.com/depot/G34-Water-Flow-Sensor-p-1083.html
thx