On my current setup I chrono 250 and this calculator gives me 253. I use my OT2 "adjusted" IBO number.
I find with any of these calculators, with a particular bow, if I weight and check the actual specs I can adjust the IBO so the speed agrees with my chrono. Once done I use this adjusted IBO if I change DW, DL, Arrow weight, the calculator is pretty close to the actual chrono data.
I use both OT2 and AA with great satisfaction. If folks blindly input manufacturers IBO it's a crap shoot if the calculators work or not. As they say, garbage in, garbage out... There is a good chance the error is the IBO nuber, not the calculator.
I could care less about speed but I use this same method with DW, and arrow weight, looking at other calculations on unfamiliar set ups.