Since this route assesses your FTP, do you need to have your trainer in ERG mode for it to work accurately? My first guess is no, but there is still a little neggling question mark in the back of my mine I need to kill. Thanks
No. Not least because you just free ride it and there’s no Erg when free riding 
Hi @Scott_Selkirk, welcome to Forums. This is Juan from Zwift.
I understand how important it is for you to clarify your doubts. As David mentioned, the ERG mode is not necessary. It’s basically an in-game FTP test. You should push as hard as you can on the climb and Zwift will detect your FTP based on your segment time and average power.
I would like to suggest you take a look at this Zwift Insider article to learn more about it.
Ride On.
ERG mode in an FTP test is only used in ramp tests. In a normal 20 minute test you would not be in ERG mode during the 20 minute effort. However if you want to achieve steady pacing on The Grade, you can move the Trainer Difficulty setting in Zwift to zero so you can spin a comfortable gear and maintain power without shifting when the gradient changes.
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Thank you. I’ll give it a try that way. I don’t think I’d be able to do it at 100% with an FTP of 140w/kg and a max HR of 150. I would run out of lungs, legs, and balance way before the top.