looking at arrow eight, you will have to ask yourself the question if you are a 3D shooter or a hunter.
For Hunting arrows you would like a higher weight and a higher FOC.
For 3D arrows you would like a lower weight and a lower FOC.
For 3D I use a 9% FOC and an arrow eight of 8,5 to 9 GPP.
As already told here, you don't want to go under the 8 GPP. For your setup this means the following:
You shoot 40 lbs at 28" but you draw 31 inch. The poundage of your bow at 31 inch will be around the 46 lbs (5% per 1 inch drawlenght more)
So this means that your minimum weight should be: 46 times 8 = 368 grains.
The ideal arrow weight also depends of your bow: some bows can be shot with low-weight arrows, but some other bows really like heavier arrows.
When an arrow is to low in weight, you can feel this, because the bow will be less quiet, give more handshock and/or vibrations and will be less foriven.
Just for finding out what weight your bow prefers, I suggest you shooth carbons with some different weights. Easily done by different points and/or weight adaptors. Try to make some different weight setups, such as 9 GPP, 10 GPP and 11 GPP. Feel how your bow reacts by different weights. In this way you can detect the limits of your bow fast and with what weight your bow shoots really nice. Then the tuning begins ofcourse...