Personally I up the incline to 1% for just distance, tempo speed work. Obviously changes if I'm doing hill work.
I don't modify distance or effort going from road to treadmill, BUT not all treadmills will read the same distance and they won't match up if you have a watch that can track the run indoors.
A foot pod helps to be more accurate.
In may case, I go with what my foot pod distance and speed is unless it screws up running me at 3 min/mile.
It tends to get less accurate in speed intervals.
Worst case I calibrate the workout to the treadmill distance if the watch gives me something completely inaccurate.
Most of my workouts are based on time duration aside from long runs, so how far isn't as critical.
Ryan http://www.SetThePaceTriathlon.com http://www.TriathlonTrainingDaddy.com Sample 70.3/140.6 Training Plans