I have an Arione on my Cannondale and have had to experiment with different angles because its shape was so different to my previous saddle.
Also - the Arione is a very long saddle. In fact, Fizik had to get permission from the UCI for it to be used in races because it exceeded the then norms. You may find that you can't use measurements taken off your other bikes/saddles to determine its fore-aft position because of that. In fact, you might have to completely rethink your saddle settings.
I was once on a training camp near Benidorm when former pro Marcel Wüst was one of the guest riders. I limped back from one long training ride and said hello to Herr Wüst, who was sunning himself on the lawn outside his room. He took one look at my bike and leapt to his feet, declaring that my saddle angle was "not correct" - very German! He insisted on levelling it so that the back and front were at the same height, but as you point out slow down - that makes the front part slope upwards. I only managed to ride about 1 km with the saddle like that and I got the dreaded numb nuts syndrome. I adjusted it back to my preferred 'front section level' setting and had no further problems with it.
Start off with 'pretty level' and try angles above and below that. There might have to be a compromise between seating comfort and arm/shoulder comfort. I find the saddle more comfy when sloping down slightly, but then I feel as if I am sliding forwards and have to push back with my arms. I agree with shooter560 - go with what angle best suits you.
