85% of my best 5 min power this year would be 286W which I know I can't possibly hold for a hour. That's how I would pace a 20 minute max power effort.

My guesstimated FTP which I use for training zones and pacing is 260-265W, closest to a 1hr test was a 246W average for 1hr 17 min over a route with some downhills and the like.

78-79% of 5 min max would be a good estimate for me.