Ma you also have to include arrow length into the equation. A 500 spine arrow at 26" in length will act stiffer than a 500 spine arrow at 30". And like Roger stated above the tip weight also comes into play.
Using the chart above and your arrow length is 28" using a 100 gr tip, for 45-49 lb draw you're at the high end of a 500 spine arrow. Personally I would opt for a 400 spine. Three reasons: 1 if you so desire you can crank up the draw weight all the way to over 60# should you choose; 2. The stiffer/heavier arrow will be easier to tune, quieter to shoot, and have less stress on the bow and 3. Will give you added spine should you decide on going to a heavier tip.