Years ago when the BHA started posting rail movements on their website the horror of the distance changes stuck home.
www.britishhorseracing.com
For example, due to rail movements, Fakenham would have distance changes of up to +258yds and more recently Wetherby topped this with a distance change of +300yds.
For the NH I divide the distance change (rail movements) by a constant of
15, so +258yds (17.2s) and subtract this from the comparison per furlong time i.e (slow by 48.0s) and if the distance is short -108yds (7.2s) you add it to the comparison per furlong time.
On the flat, I now use a constant of
13.
To explain the constants, over the jumps a horse travels an average of 15.0s per furlong and on the flat 13.0s.
I am the originator of this method.
Mike.