As far as the effect of shooting arrows with low foc, I think the best way to describe it is,it is like shooting straws , there is a lack of sense of direction, nothing up front to keep the arrow on track with.
I have never really calculated my foc on my arrows but feel it is more important to know what pt weight will work for you and the shafts you are shooting, and the round you are shooting.
For example for the 90 meter stuff I think you will find that a lot of guys shoot between 100 and 110 grains, this just seems to work, now alot of the 3- d guys lighten up a bit to say 70 to 80 grains for the speed .
I would just shoot the pt weight that gets you the best results and dont worry to much about the speed, you can always turn your bow up a couple of pounds.
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