CAmo,
This would be better answered in the Pow Wow section as it is not a form question.
I think you are confusing arrow SPINE (measured in inches of deflection like .500 or .600 inches) and arrow weight (measured in grains). Shortening a shaft will increase the spine (make the arrow stiffer) and make the weight less. I think you could start with a .500 spine and then adjust AFTER you shoot for a while.
Arne