You may be tempted to think that if the spine you get for your arrow on the spine tester is 50# that your arrow will be perfect for your 50# bow. But you would be mistaken to make that assumption. There are a variety of factors that influence what arrow spine will work best with your bow, including the length of the arrow, your draw length, the speed of your bow (and therefore the type of string, whether it's a longbow or recurve, etc.), the weight of your point, how close the arrow shelf is cut to the center of the bow, etc.
The spine tester measures static spine. That's just how much the arrow bends when you hang a weight from it. But there is a thing called "dynamic spine." That's the effective spine of the arrow when shot from a bow. The best way to find out what arrow spine works best for your bow is to shoot a variety of arrows, each the same length from the same bow drawn to the same draw length with the same weight on your point, but with different static spines. (Three Rivers Archery sells a test kit that includes four arrows with different spines, so you can experiment. An alternative is to make your own arrows.) Calculating the dynamic spine will give you a closer estimate of what spine will work best so that you don't have to do as much experimenting.
There are some thumb rules that will help you estimate your dynamic spine.
The spine tester assumes your arrow is 28" long. The shorter the arrow is, the higher the effective spine, and the longer, the lower. The thumb rule is 5# for every 1". So, if your dynamic spine is 50#, and your arrow is 29" long, your effective spine (accounting only for arrow length) is 45#. If your arrow is 27" long, your effective spine is 55#.
I have read this in a variety of sources, including The Traditional Bowyer's Bible, Volume Three, p. 228-30. In what I have read, it's ambiguous whether this thumb rule applies to arrow length or draw length or both. After all, it's possible for a person with a 28" draw length to shoot a 29" arrow or a 30" arrow. Likewise, it's possible for a person with a 30" arrow to draw it to 28" or to 29". So I dunno. But don't stress out about it because we're only trying to get a closer estimate so we can narrow the range of arrows we should test to find the perfect spine.
Weight of point
There seems to be some disagreement out there on what the AMO chart assumes the tip weight is and how much tip weight effects dynamic spine. Most simply say that heavier points reduce spine and lighter points increase spine, and they leave it at that. They don't give actual figures. Gabriela Cosgrove, in her chapter on "Wooden Arrows," in The Traditional Bowyer's Bible Volume Three, p. 229, seems to think the spine tester assumes a 110 grain point. She says, "Add 5 lbs for heavy broadheads(those over 110 grains)." Other sources (which I don't have the energy to google at the moment) seemed to think the spine tester assumes a 125 grain point.
Stu Miller came up with a dynamic spine calculator where you enter all your variables into an Excel spreadsheet, and it calculates your dynamic spine. I used his calculator to figure out how much point weight affects dynamic spine. I began with a static spine of 55# and typed in different point weights according to what is commonly sold in archery supply stores, then calculated how much weight you'd have to add or subtract from your static spine to get your dynamic spine. I came up with this chart.
Speed of bow
Grabriela says that we should add 5 lbs for extra fast recurves (TBB, Vol. 3, p. 229) Unfortunately, I was not able to find any charts that equated speed of bow with difference in spine. But again, we're just estimating here, so no worries. We just need to know that the faster your bow is, the lower your effective spine, so Gabriela is telling us to add 5 pounds to whatever our static spine is to get the spine we need for our faster bow. It has to do with the laws of inertia.
Degree of center-shottedness
I did find a chart that equates distance from the center of the bow to difference in spine, but I don't want to look for it again. Maybe I'll edit this build along if I come across it some other time. But suffice it to say that the closer to center shot you are, the stiffer your arrow should be. The wider your grip, the lower your spine should be.
Let's do an example. Let's pretend like we want to estimate the spine we need for my favourite bow, which is 52# at 28".
We begin with a static spine of 52#. I like to shoot 125 grain points, but let's pretend like I'm going to shoot a 100 grain point. If you look at my chart above, you'll see that using a lighter tips effectively stiffens my spine by 7 pounds, so I need to subtract that 7 pounds from my static spine.
52# - 7# = 45#
I'm going to shoot a 29" arrow. Since lengthening the arrow by 1 inch effectively reduces spine by 5#, I need to add 5# back to what I have so far.
45# + 5# = 50#.
Since I make wicked fast bows, my effective spine is going to be weaker by 5#, so I need to add 5# to my spine to make it stiffer.
50# + 5# = 55#.
My bow is close to center shot, but not quite there, so I think I'll just subtract 2# to make the arrow bend better around my bow.
55# - 2# = 53#.
So the perfect spine for my 52# bow, given my variables, is probably close to 53#. But this is a very rough estimate. I still have to experiment to really find the perfect spine for my bow.
How spine affects arrow flight
The spine of your arrow will determine whether it shoots where you're aiming or not. It's different for a left-handed shooter and a right-handed shooter. If you're right-handed, and your arrow is too stiff, it'll shoot to the left of where you're aiming. If it's too weak, it'll shoot to the right. If you're a left-handed shooter, and your arrow is too stiff, it'll shoot to the right of where you're aiming. If it's too weak, it'll shoot to the left.
Here's my little illustration about how spine affects arrow flight.
You can use this to determine whether your arrows are too stiff or not stiff enough. So experiment and find out what spine of an arrow shoots best with your bow.
When you buy shafts, they are usually grouped in 5# increments, e.g. 45-50#, 50-55#, etc. 5# makes little difference, so as long as your arrows are within 5# of each other, they should all shoot pretty consistently.