Creature Harvesting 7 (Source)

Check again Dave. It's F(1+hm/100). Also, isn't a master ranger's harvesting mod +150? As I understood it, they increased the bonus in the master ranger box from +10 to +60, making the new total +150 (unless you're a trandoshan). Also keep in mind that veghash gets added directly to hm, but the grouping bonus is applied at the end. Also, as far as I know, the grouping bonus does not work with harvesting droids. So, if you were using a droid, then you were not getting the 20% bonus.