Looks like you've not read my note properly. Mylnes Culvert, runs from Stone Bridge beneath Quay Head (southern tip of Electricity House), around the EAST side of St Stephens Church (there's a curve here) then following the course of Marsh Street to Prince Street, thence picking up the route you've mapped. Also from Stone Bridge there's a stormwater culvert running beneath the centre to No.1 Harbourside bar, what many Bristolians wrongly believe to be the Frome outlet. You've conflated the two culverts. Hope this is now settles the matter. I've been studying this topic for many years and can provide evidence if you need it.