Hi there,
From my encounter, if the Jet core is reported to be at FL 350, usually, plus minus 2000ft from the core is relatively calm, if it might be quite turbulent when passing thru 4000ft below the core all the way to around FL200.
The best way to enter a JET stream is to go at best ROC to get pass the turbulent region as fast as possible.
Usually, the sigmet charts will indicate where the CAT regions are. The CAT regions are way too big as well as too vague to avoid. Only thing to do is to prepare the cabin earlier (PAX a/c), complete all paperwork and get ready for the turbulent times. Sometimes, the CAT might not be there at all
Just my five cents worth, hope to hear more from the experience guys,
Cheers,
lion-g