When I measure my arrows using Beman's recommended method (from nock groove to end of shaft) I measure 29 1/3". Now my question is that when using an arrow selector chart, do I round up or down to get the best shaft? The reason I ask is because if I enter 29" into he selector it suggests a 340 (what I'm shooting now) shaft however if I enter 30" it suggests a 300 shaft. I can't help but to wonder if I'm slightly underspined with my arrows.