For a turbo accuracy is less important than consistency. Who cares if the number reads 5% less or 10% more than if it does this consistently on every ride. The problem comes when it reads 5% over some of the time, then 10% under, etc. Any attempt at working out your trend over time is going to be lost in the noise.
I have power meters on two bikes but don't bother with a turbo (mostly because I live in a 1st floor flat). If I want a guess at my FTP then I can use a Wattbike at the local gym. And I find a ramp test much easier than a 20 minute test. Ramp tests may not be as accurate as a 20 minute test but they're a lot easier to do physically and mentally. There's no pacing involved (you don't have to guess at what power you can maintain for 20 minutes and then attempt to do that, you simply follow the protocol until you can't go on any more). It's also shorter in time. I can warm up, do the test, cool down and be done in under 20 minutes.
This lockdown is going to be an interesting experiment as I'm off the bike completely (last ride was March 13th) although I am doing lots of running and core/leg workouts so all is not lost. I'll do a ramp test as soon as the local gym opens again and then start cycling and weekly spin classes to see how the FTP progresses.
Goals for the next few years involve rides around 100 miles (the Prudential 100 to finish off my London Classics medal, and half and possibly full Ironman which I've been talking about for years). Those distances (and longer) haven't been a problem in the past but doing them fast rather than at Audax pace will be the challenge. I'll use 200km Audaxes as training for them and try to ride them faster and with minimal stopping.
Knowing my FTP doesn't have any practical use for me really as I don't use it for any type of structured training. For my local spinning classes it's useful to know what I can hold for various durations (30s, 1m, 5m) which is all related to FTP but those can be often found with trial and error, as can intervals at the gym as I just set myself a target for, say, 3x8 intervals at 280W and then record them on the Garmin and analyse them after the fact to see if I'm improving and need to raise the target for next time (because they felt easy or HRmax is dropping and there's spare capacity).
I may just save up for a Wattbike (I'd rather have a whole exercise bike rather than a turbo, more compact and less faff, and my wife and daughter can use it). I guess there are going to be a load second hand coming on to the market once lockdown ends. If I had that I'd then be able to do some proper structured training on Zwift/TrainerRoad/BigRing/etc and do it from home (and when on boring conference calls at work) which would probably get me strong/faster a lot quicker than my own ad hoc approach.