I think you think I'm thinking the same way as you but I don't think I am. Note in my quote the word equivalent as opposed to something less meaningful like average. The figures I obtained were in a fairly intense setup which didn't just take the lengths from the winner against track going in isolation. It was a sort of best fit affair using many other variables with successful handicapping being the measuring stick. I know this all sounds very wishy washy but it's pretty hard to explain without producing a white paper on the whole event.
Bottom line, I believe there is some difference in margin from the winner between the same two horses running on a dry and a wet track disregarding other factors like track preference and the like.

KV