Me no grass drag expert
I had red cat primary,yellow sec and 55/53 before and was reving 8700 which is 300-400rpm too high
engaged 3700 with red primary spring
I now have yellow/green Cat primary spring,green EPI secondary(108LBS) and 57/51
engage 4700
I liked that set up but my weights were way too light I had 52 or 53G and was over reving to 8900
yet I still got over 100mph on the dream meter on a ice covered road
stock gearing 20/39 and 96 studs
might try 18 tooth and 56gram weights to bring my RPM down
most of my riding is in the 40-70mph range
+ I am getting a Ripsaw

Sorry for babbling

as you are looking for grass drag set up and not trail
There must be someone on here that grass dragged the ZR600s before