Which Saddle?

Still struggling with the Arione I fitted to my steamroller. I went for around a 20 mile ride last night and it started getting uncomfortable after about 10. What I did notice though was that if I sat further back on the saddle it immediately felt much more comfortable. This got me thinking that I may have positioned the saddle wrong. I measured from the tip of the old saddle to the stem and replicated this measurement on the new saddle however I recall reading somewhere that the Arione is a longer saddle. I have moved it forward around 10 mm but have yet to give it a try.
Out of interest, what is the usual method for positioning a replacement saddle?

Sort of what you've done. I usually measure nose to (some point of) bars, plus BB to top, half way along. Also replicate angle with spirit level laid along it's length.

But then you have to experiment/estimate differences based on saddle length, sag/squish, location of main sit-point, curve/profile, etc. Inevitably there will be further fettling after the initial fit.

I moved the saddle forward around 10 mm as mentioned above and finally got to give it a proper test yesterday. I covered around 35 fairly wet miles wearing exactly the same clothing as had been worn when finding the saddle uncomfortable previously.

This time around I'm happy to report the saddle was much better. Not perfect but that small adjustment made a huge difference.

Moral? Position is probably as important, if not more so, than the saddle.