The thief class skill “Steal” has a tooltip of 17.5 seconds with 300 points in trickery, and using the trait “Slight of Hand” (20% steal cd reduction and 1 second daze on steal).
20% + 30% = 50%
0.5 * 35 seconds = 17.5 seconds (matches tooltip)
When you activate steal and then the ability you use in rapid succession (to see the cool down on steal), you see the cool down is 21 seconds, not 17.5.
If you apply the cooldown reductions in sequence:
0.8 (20%) * 0.7 (30%) * 35 seconds = 19.6 seconds.
This does not match the tool tip, or actual cool down timer.