if you look at a bow.. the little bar with the cable slide, the distance from the handle to there is the brace height. a longer brace height is said to give more shot forgiveness when you torque a bow. although, i went from an 8 inch brace height to 6 inch and have no noticed much difference. im not really sure what the back bone is refering to, and as far as the angle of a shot.. there is not much need for a range finder with arc, unless you are shooting some extreme angles or a slower bow.. you can figure it out with a simple calculation..
heres a comment i posted to someone else when asking about a range finder

RE: rangefinders - 7/18/2008 9:51:11 AM
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mhouck06
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someone else had a post like this about a month or so ago.. heres what i told them
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its not worth getting the range finder.. if you measure off around your stand all you need to do it shoot that distance.. the distance from the base of your tree to the target, no matter how high you are, it works. if you dont trust that.. a simple math equation will do the trick.[/font]
[font="times new roman"]A^2+B^2=C^2 [/font]
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just take ur stand height as A in feet, ground distance as B make sure if its 30 yards you enter 90 feet and then add them together.. take the square root of the sum and then divide that by 3 to get ur distance from stand to ground[/font]
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ex. 20ft stand. ground distance, 30 yards[/font]
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[size=100][font="times new roman"]20^2+90^2=8500 [/font]
square root of 8500 = 92.195 ft.
92.195 / 3 = 30.7 yards [/size][/font]
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as you can see, the difference is only minimal and would be hardly noticeable if you took that shot for 30.
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