Many thanks for all the replies but I am still not clear if I can achieve what I am looking for. My original post was perhaps not detailed enough so I will try to explain in more detail. Again many thanks for your patience.
My current set up is Arrow length from nock throat to end of shaft 28 1/8" ( Sorry for earlier error) and they could be reduced by 5/8". My arrows are
ACE,s 720s. My limbs are W&W Ex Power 30lb @ 28 ". My limb poundage is set at the middle position. I am shooting at a decent 1st class level and want to achieve Bowman but @ 90m/100yds I am at the extreme of my set up and my "groups" are very wide in the bottom half of the target and the green. OK I could aim above the gold but find this unsatisfactory. If the arrow is shortened by 5/8" this will stiffen the arrow and if the poundage is increased this will weaken the arrow The question is by how much to keep the same balance. Both actions should increase the arrow speed and move the groups higher and tighter. Is my reasoning OK or might X10s be the answer. Using higher poundage limbs is not an option.