That depends on what arrow length you will be using.Typically,350 fps dictates a 30" or greater draw length and an arrow at least 29" long.A longer arrow flexes more so it needs to be stiffer.

NOW,the confusing part.Spine is measured in deflection,which means the higher the number,the more it flexes.Typically a .340 spine is how much the arrow flexes when measured at static.BUT and this is a huge but()some manufactuers,for whatever reason thought it would be a good idea to size their shafts opposite of deflection.Easton arrows that have .400 on the side of the shaft represent actual deflection but carbon Express will have 350 that correlates to nothing and their higher numbers mean stiffer.


So to answer the question,a really stiff arrow is needed for a bow that shoots 350 fps and typically the arrow is longer and this means it should be even stiffer so most of the time,a .300 spine or stiffer is needed for a bow that shoots that hard.