I’m not sure if I understand the question fully but if you’re trying to figure the anulus width you need with inner radius (r). You have the outer radius (R) and the chord length (let’s call it 2L).
So if you draw the radius lines from the centre to the ends of the chord you have right angled triangles where the hypotenuse is R and the opposite side r and the adjacent side is L. So its just trigonometry.
Working it through if big circle is diameter 10 then R = 5
Chord length is 8 then L = 4
Then inner circle radius is r = sqrt(R^2-L^2) = sqrt(25-16) = 3
So the anulus width is R-r = 2.
Was that the question? 😀
This obviously assumes the chord is tangent to the inner circle which I’m unsure from the question whether this is the case…