Something seems off on the increased IBO speed. You shouldn't need to adjust the IBO speed of your bow. It is what it is. Just change the input numbers for the heavier arrow.

I've had the same thing happen to me with OT2. I was checking about 4 different arrows. The speeds were all over the place and then realized thatI needed to reset my basic inputs and then it would work fine.

The 500 gr arrow with a 12% FOC isn't to hard to build. Just get an arrow with camoon it that weighs about 12.2 gpi. Blazers, and about 29.5 inches long.