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A pipe running full has hydraulic radius D/4, so Manning gives velocity directly. A 300 mm concrete pipe at 1 in 200 carries 0.062 m3/s at 0.88 m/s - just under the 0.9 m/s self-cleansing threshold, so a steeper gradient would be specified to keep solids moving.
Full bore hydraulic radius
R = D / 4 for a circular pipe running full
Capacity
Q = (pi D^2 / 4) x (1/n) x (D/4)^(2/3) x S^(1/2)
Because at about 94% depth the hydraulic radius peaks. Capacity there slightly exceeds the full-bore figure.
Commonly 1 in 80 for foul drainage to maintain self-cleansing velocity at low flows.