The correct arrow length is 1'' in front of the rest. Taking this into consideration you have to calculate that when making your selection. Length will affect arrow spine. The 400 rating is only at a given length. Therefore you must go with that suggested weight and play around with draw weight to match it to your arrow spine atthe correctlengtharrow length for your setup.

I am no expert here but this is what I have gathered from otherthreads similiar to this. Maybe someonewith more experience can axpolain it better and more clearly than I.