Ihave used these calculators several times since the year 2000, when I shot for Pearson. Back then I did the shoot 20 & 40 chart.The one day I was havin a very good day of shooting(sub2" groups at 40)were about 16" lower than those at 20 yards. Using the chart that would calculate to about 294-295. The bow in question chrono'd 264 on three different chronographs.
This leads me to believe one of two things. Either the calculators are designed to make us feel good (ego thing) or the bows IBO ratings are greatly inflated. I'm inclined to think it's a bit of both.
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin/Rytera Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver