the 340 has to do with the spine stiffness which is the deflection in inches represented .340" and im pretty sure thats not true about that being the lightest arrow you can shoot. i have shot the 500 lightspeeds out of my bow which are 6.5gpi on 70# and a 29" draw. so for example there should be no problem with your bow if you wanted to shoot the 340 flatlines wich are 8.2gpi to pick up a little speed. Some arrow websites have an arrow chart that says what arrow to use on this many pounds with this number draw length so you can get an idea of what arrow to use but i bet thats what they are basing it off of.