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Crosstalk falls roughly as 1/(1 + (S/H) squared), which is the basis of the 3W rule: spacing three times the trace width, or about three times the dielectric height, cuts coupling to a few percent. Backward crosstalk saturates once the coupled length exceeds half the rise-time length.
Crosstalk
Kb approx 0.5 / (1 + (S/H)^2); saturation length = rise time x propagation velocity / 2
Keep centre-to-centre spacing at least three times the trace width. It is a proxy for the spacing-to-height ratio that actually governs coupling.
Once the coupled length exceeds half the rise-time length, additional coupling arrives after the reverse wave has already reached its peak, so the amplitude stops growing.