Sometimes I want to hop on my bike and make up a workout as I go. Some apps allow manually setting a target wattage. That’s a fine feature, but we don’t really need it. Here’s what I do:
Make a new workout on the Xert webpage. Add a bunch of one-hour blocks at 100% Threshold Power. Call it “manual”.
Select your new manual workout on your mobile app or garmin and set the trainer to erg mode. Start riding. Adjust the intensity. Now, if you adjust the intensity to 75%, you’re riding at 75% of TP. 120% intensity and you’re at 120% of TP.
(Note: my “manual” workout s actually a bunch of blocks at LTP rather than TP, since most of my manual workouts are done near LTP. This saves me clicking intensity down a bunch when I start the workout.)