After playing with the OnTarget software for the past week, I'm convinced you cannot get a truely accurate recommendation unless you can manually put in the speed (obtained from an accurate crono) and then adjust the IBO speed until the calculation speed matches the manual speed. Without doing this, program recommendations were way off. It also helps to actually weigh the arrow, especially if you're doing something like added wraps or have fletching that you don't know the weight.

When you add the crono'd speed, be sure to set the program up with the arrow stats for the actual arrow you shot through the crono. Then adjust it for the arrow you'd like to shoot.

DMP, I highly doubt your bow is shooting 273 fps.