Hi mcgiever
It's not an exact science unfortunately. I've been in the silky smooth core of a 150kt jet stream, and also been bumped around the edges of much smaller ones.
From our Lido explanation:
"Shear rate.
Vertical windshear value in metres per second per thousand feet at the current position. The SR value is determined by quantifying the difference in wind speed and direction for the wind 2,000 feet above and 2,000 feet below the planned altitude at the current position using the ??? Airlines vertical windshear algorithm.
NOTE:
SHR is to be used as an indicator of the potential for Clear Air Turbulence (CAT) only. The higher the SR value the greater the difference in wind speed and direction between the two altitudes. No studies have been carried out to validate the relationship between SR values and the probability and severity of CAT. This information is to be used as guidance only and must be correlated with other available information such as SIGMETS and SIG WX Charts."