Steve, it occurs to me I have another query. Judging from the photo the pull on the rod tip was from upstream, ie you didn't have much of a bow in the line. Did that mean that you were able to clip up? I've found that can be a problem as you need to let line run off the spool if you have a big bow in the line, which is how I have read about upstream feeder.
I was clipped up, John. I wasn't needing to let line out to form a bow, the movement of the feeder downstream was allowing it to form naturally. That probably gave me less accuracy than doing it explicitly but it was landing in the same spot and then moving downstream until it reached the right balance point, so probably ending up in more or less the same spot.
I think it's probably a matter of trial and error, balancing weight against flow, and it's probably harder to do when the flow is greater - this was the Thames in summer, so slow and steady. Having said that, I have used the same technique many years ago for chub and barbel on the middle Severn where the water was four feet deep and very fast and the feeder was a big Drennan blockend.