I guess the real question now...is whether you can alter the output of crops...the CLUSTERSIZE token only mentions the amount of plants you get from herbalism
I believe the CLUSTERSIZE token also affects crop yield, but don't quote me on that.
If it doesn't, though, there's a sneaky sneaky workaround you could use to artificially decrease crop yield. If a plant has multiple usable parts, it has an equal chance of dropping each; but the measure of "usable" is just whether it has any reaction products or is made out of a [STOCKPILE_PLANT_GROWTH] material. So, by adding useless growths of a junk material that's got a reaction, you can effectively reduce the crop yield by a significant proportion. You can keep the extra growths from showing up on wild plants by giving them [GROWTH_TIMING:0:0] .
You'll need to decide on a way to compensate for the reduction in seeds - possibly by making the reaction on the junk material produce seeds, or maybe by sending all successfully-harvested plants through the Quarry Bush reaction and just upping its seed output.
If you decide not to convert the chaff to seeds, you can define the material so that it'll dissolve into nothing as soon as it's picked using the heatdamage and colddamage material tags. No fuss, no muss!