You really don't even need to bother trying to convert speed to watts... If a hard 20 min interval is 20mph then just do 20mph intervals.

Most of the time the resistance from trainer to trainer will not be the same, or at least not close enough to get a real power reading.

For example Cyclops posted that their Fluid2 Trainers are "ride tuned" 400 watts @ 25mph.

On the one I have this is not true. ~22.5mph is 400 watts. Cyclops Fluid 2 Review