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I assume that the Rn values are just for the arc portions. I also assume that this curve is through exactly 90°.
The circumference at R2 is πd, which is π(1.5)(2). Dividing by 4 gives the length of the arc. Assuming that the radius at R2 is in cell A1:
This could be arithmetically simplified if you wish, but I prefer to show the unreduced form to suggest the original logic.
At this point I think your model breaks. You said that the center line radius is 1.5, and that D1 is 6. If that is correct then this drawing is not at all to scale. In this drawing the center line radius is about twice as big as D1.
But here is the math. Let's say the value of D1 is in cell D1.
For R1
where 6/2 is half of D1, since that is the difference between the center line radius and the inner radius
For R3
similar logic
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