FindFit[{{16.76, 10}, {14.31, 20}, {12.08, 30}, {10.35, 40}, {8.92, 50}, {7.55, 60}}, {(arctan(B*v) - arctan(B*19.44))/(A*B)}, {{A, -0.2}, {B, 0.2}}, {v}]

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