I’d err on the side of low. If your FTP is too high and you try and try and try to train well above capability, you’re going to fail workouts consistently and risk injury. Failing workouts wears on your confidence and mental state. When FTP is too high, you just bludgeon yourself every time you get on the bike.
When it’s too low, and I assume we mean 5% or so, you’re still getting generally the same adaptations in most cases, you’re able to handle the volume, and you’re still getting fitter, just not as fit as you would if it were accurate.
The ramp test has given me good training baselines. I had my FTP too high for a little bit, and it was misery. I’ve backed off a touch (2.5%) and am training base again to get a better foundation at these higher levels (13% higher than I started the season.) I may be training too low, but I always have the option of dialing up intensity if things feel too easy. That always feels better than the alternative.