assistance with addition

[Sockrsblur]

I saw a topic not to long ago about total arrow weight. Not having a scale can someone walk me through some simple addition? I shoot a 29" Beman MFX Classic 500. It says 9.7gpi on it... Grains per inch? if so that would be 281.3 grains + 75grain HIT insert + 150 grain broadhead = 506.3 grains?! Is it that simple to estimate without a scale?
Thanks for the math lesson guys!  

[awbowman]

Add 12 grains for the nock, estimate wrap if any, plus fletching.

Both wrap and fletching will vary based on length

[Sockrsblur]

thank you fred!