The arrow's spine number (300, 340, 400, 500, etc . . .) simply means that when the arrow is placed in a testing device supported at two points 26 inches apart, and a 2 lb weight is applied to the midpoint of the arrow shaft, the amount of deflection as measured in thousandths of an inch is the arrow's spine rating. So if an arrow deflects .400", it's a 400 spine weight arrow. GPI is "Grains Per Inch". Your Easton Power Flights weigh 8.4 grains per inch. There are about 15.5 grains in a gram. The less deflection (lower number), the "stiffer" the arrow.
That is called a static rating. In a dynamic environment (arrow actually being shot from a bow) two other factors come into play. the weight of the arrow point, and the amount of force pushing the arrow (draw weight and to a lesser extent cam "aggressiveness"). If there is a heavy weight up front, and a lot of energy pushing the arrow, it will flex more as it comes off the bow than an arrow with a light point and less energy.
So, how do you order an arrow using the Easton arrow size chart?
First figure out how long your arrow need to be. This is necessary because the dynamic spine of an arrow will change with it's overall length (with alike points - a longer arrow will flex more than a shorter arrow). Get a helper with a Sharpie marker and have him/her mark where on your arrow touches the arrow rest when you are at full draw. Measure from the nock groove to the mark. Add 1" to this number. Let's say you got 27.5" at the mark, add an inch and you're at 28.5".
Next, figure out what cam you have. In my opinion, this doesn't make a huge difference. Very few hunting bows have soft cams, consider yours Medium or Hard. On the Easton chart, beneath the Cam selection, they list a range of bow speeds. Look up your bow's IBO speed and see if it falls into the "Hard" or "Medium" range. I'd guess yours is a "Hard" cam. Now go to the Easton Target Chart (or Hunting Chart, but for this example I'm using the Target Chart). Under the Hard Cam column, move down until you get to the 55-60 lbs row or the 60-65 lbs (I'd use the 60-65 lbs range as your bow is probably 60-70 lbs draw weight range). Now, move across until you get to the column under the arrow length (in this example 28.5"). It says "T10".
Now find the Group T10 arrow list and that will give you a range of Easton arrows which will be properly spined for you bow. Keep in mind that this is a range with the "weakest" arrows at the top of the list, and the "stiffest" arrows at the bottom (they are also separated with carbon on top and aluminum at the bottom). The arrows followed by the letter "R" are recommendations for recurve bows.
From the chart, 380-440 spine is correct. Easton breaks it down by their arrow line-up, so you'll need to decode which Easton arrow is listed. Their economy arrows aren't listed, so your Power Flight's aren't listed specifically, so just go by the generic 380-440 range. Your 400 Power Flight's are right down the middle.
As you get more experience, you can experiment with changing how an arrow tunes by changing point weight, or arrow length. It may take some pondering before the relationship between static arrow spine rating and dynamic factors such as point weight/arrow length/draw weight become clear.