Currently the distance table of the Ashby Canal (Main Line) starts at [Ashby Canal Current Head of Navigation] and measures to [Marston Junction]. Is it possible to reverse the distances to start at [Marston Junction]. I ask this as the [Milepost - Marston Junction 14 Miles / Bath Yard Basin 16 Miles] needs to be in a fixed point relative to [Marston Junction]. Whenever the [Ashby Canal Current Head of Navigation] is extended the milepost moves relative to the that position.
Good question
it's definitely something I've thought about and, as you say, it makes good sense in this case (and having the direction of a dead-end waterway coming from the junction would be entirely reasonable anyway).
I have a vague memory of doing it semi-manually at some time (and there's a telling comment in \"edit_waterway\" about allowing you to set the distance of a waterway that suggests I was planning to build it in), but a quick code-search didn't throw anything obvious up.
I'll dig further and either do this one by hand or add it as a mode for edit waterway - whichever turns out to make most sense.