That's interesting (the study above).
You're honestly going to get so many different responses on the treadmill. Some people swear it's harder for them. Machines vary and mental interest/willingness to run on them varies too.
I'm going to agree with the general posts here so far. I did almost all of my running on the treadmill over the winter, at zero percent, and estimated a race pace that was basically 1 second off on the roads.
There is also a reason why you see a range of "easy run" or general running paces in training manuals. It doesn't need to be exact. And it's pretty hard to really replicate on a day-to-day basis. The roads aren't flat and that 1 percent doesn't really replicate them. There's changing elevation all over the place, at least where I live, and even some seemingly minor hills are going to throw the pace off if you're truly going by effort. Even if you're on the track, sometimes you might be tired and you're 20 seconds off the usual easy pace.
Just run a time trial on the treadmill, adjust to that metric, and you'll feel it out as you go.