When you buy spined shafts the dynamic spine of the shaft is determined for a 28" arrow with a 125gr point. For each inch over or under 28" the effective spine drops or raises by 5# per inch respectively and for every 25 grs of tip weight above or below the 125gr will reduce or increase the effective spine of the arrow by about 5#. So, if you have a 30" arrow with a 150gr head and the shaft's dynamic spine is 50# that arrow will fly like a 35# spined arrow. 10# lighter because of the 2" over 28"(5# per inch= 10#) and 25grs over the 125gr standard(5# per 25gr over 125gr).
I hope this isn't too confusing. I draw to 26" but cut my arrows to 30". This gives me a weaker spined arrow but with a little figuring I can make the arrows fly well. I like a longer arrow because I think it gets around the bow better.