Just jump on and go, and you’ll see that you don’t need to worry about this with erg mode on. And it will allow you to shift the % bias on your FTP if it’s a bit too hard or too easy for the day. The erg mode is not so crazy that you can’t fluctuate a little. I usually shift the %bias via the companion app, but I’d guess you can do it elsewhere too.
If you’re controlling a smart trainer and following the workout (e.g. from Zwift, directly from your Garmin, etc.), your computer/device will use erg mode by default. This means that it will force you to do exactly the prescribed power. For a range, I believe this means the exact middle of the range. In this mode, don’t think about it, just spin! Enjoy the satisfaction of hitting that TSS target perfectly (+/- a few).
What I do sometimes is follow the workout on my Garmin but play on Zwift. Zwift controls my smart trainer and gets the power numbers. Garmin will still tick along keeping track of time. This is like doing a workout outdoors, basically. If I’m looking for z2 cruising with sprints, I ride Tempus Fugit. If I’m looking for sweetspot intervals, I go up and down Alpe du Zwift. In this style, I can make use of the whole % range .