The calculator takes into account more than just the advertised weight of the bow. Things like arrow shelf cut to center or past center, arrow shelf plate material thickness, and string material, style, and strand thickness. All those things in combination work to either break down the amount of spine necessary for proper arrow flight or increase the amount needed.
The make and model of bow also makes a big difference. For example an old style Bear Grizzly with a B50 Flemish twist string won't need as much arrow spine as a new style Samick recurve with a fast flight string, even if they are exactly the same poundage.
Stu's calculator does all this calculating for you. Check it out for yourself and input the proper variables, then compare the 2 output numbers for Dynamic arrow spine, and Dynamic spine required on the bow number.
Be sure and download the new version and the instructions PDF, the instructions are very important to be able to understand how to use the calculator correctly.
It was spot on for my set up.
http://www.heilakka.com/stumiller/