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?

Placement is up to you.

But I use the point where I expect my sit bones to be, to measure both reach and height.

I have the same thing with the Gobi. If its little too far back its horrible on the road.