Constant NEAR_THRESHOLD 

const NEAR_THRESHOLD: f64 = _; // 0.0625f64

Largest moduli of the real and imaginary parts of q . q / k^2 - 1 of the zeroth diffraction order at which the reciprocal sums take compensated_threshold_distance: there the rounded form loses 50 ulps of v or more (about 3 ulps of q . q / k^2 over the difference).