There's no magic formula, you just look at the saddle and see whether it's shaped more like a T (relatively sharp transition from the *ss to the nose) or more like a pear (relatively gradual transition).
The Jett (and most of Specialized's line) is more T-shaped than most saddles out there. From what I could tell on Serfas' site, their tri saddles are too, but it's kind of hard to tell since they don't have a top view.
Definitely measure your sitbones. There's no way to know the distance between your sitbones from your other dimensions.
Serfas specs its women's tri saddle as 5.5" (140 mm) wide. Was it the women's saddle you had? Spec width on a saddle doesn't tell you everything, because some of them slope away from the center, so the effective width of the part that supports your sitbones can vary between two saddles with the same nominal width, but it's a place to start. Your Jett should have the size stamped on the nose.
I'm not a big fan of PI chamois and that seems to be the consensus around here. So yeah, trying some other shorts might be in order too. I try to match the size and shape of my chamois to my saddle.
Are you using lube? Lube shouldn't be your first line of defense against chafing, but it can definitely help, especially during the search for your perfect saddle.
Do you still have the Serfas saddle... did you try tilting the nose down just a bit?
Welcome, and HTH.
Speed comes from what you put behind you. - Judi Ketteler