As far as arrow length, I would say it's a matter of preference (to a point). I have a 29 " draw and I shoot 29 " arrows (with a QAD rest), my arrows stick past my rest by an inch or so. Now, as for 400's vs 340's vs 250's, I believe that # represents the spine. Arrows of the same spine can vary in weight (grains per inch) from one brand to the next. For example, Beman 400's are 9.1 gpi as to where Carbon Express "Maxima Hunter" 250's (which is what their chart calls for your set up to use), are 8 gpi. But the Carbon Express "Piledriver" (250's) weigh 9.7 gpi. Your arrow lentgh and draw weight will determine the spine as for weight (grains per inch which determines total weight) you have the option of going light, heavy or somewhere in between. Hope this all makes sense and, I hope it helps!