For any interval longer than 1min, i just look at avg power. It always starts high, but after about a minute, it settles in to the number I plan to hold for the rest of the interval.

TT's included. If you see your avg rising, you know you're above it, and vice versa if it's falling. I suppose if the TT had varied terrain, a different smoothing would be appropriate, but I haven't ever come across that...