There are essentially two modes (values are just examples):
<duration const="30 4"></duration>
<duration const="">5</duration>
In the first mode, we have two values (30 and 4). The 30 is the base duration value, the 4 is the Skill divider that is subtracted from the base duration, like this:
(30 - (SkillLevel / 4))
This means that the higher the skill divider is, the less the Skill level reduces the base duration.
Example of SkillLevel at 100:
(30 - (100 / 4)) = 5 seconds
Example of SkillLevel at 40:
(30 - (40 / 4)) = 20 seconds
Now this is the part I'm unsure about, but I believe the resulting value is also inversely modified by the Tool Quality as a percentage. Meaning that with a tool quality of 0 the duration would be doubled, and at a tool quality of 100 it would be unmodified. This could easily be tested and measured of course. It would be nice if someone could do that.
The second mode:
<duration const="">5</duration>
The duration is just a constant value, i.e. it takes 5 seconds regardless of skill level or tool quality.
For Uprooting, I recommend something like:
<duration const="14 10"></duration>
I hope that helps. Now I hope y'all can stop bugging me about ability durations!
(Seriously, I've had more than 10 PM's about it in addition to the posts in this thread and my Life is Balanced thread)

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Is this mod even possible? I think the problem im running into is that I cant seem to find the skill_type table mentioned in your moding guide. Would you be so kind as to post a file path? I would appreciate any advice! 
