If you’re thinking about it this much then just do the 60 minute test and be done with it. What you can then do is when fresh, do the ramp test and see how it corresponds. For me the ramp test corresponds nicely but that could also be because I’ve done a lot of top end work leading into it. It’s difficult to know just what systems are helping you get that number. You may not even care. For me, from now on I will always do a 60 minute test. I’ll know if factors are affecting my performance and not giving me a number I’d expect, it’s more of a way of clarifying what I already know.
The 60 minute test is not as grim as it sounds. Seiler suggested (it also matched my own) that if you are at (or below) 90% of HRM at 20 minutes you should be able to complete the 60 minutes. If you are way over then obviously it might be worth taking a recovery then going another 20 minutes to complete a good quality threshold session.
Remember, doing those 3 tests could yield different results for many other reasons (motivation, stress, fatigue, nutrition).
Having said all that, if you’ve been doing some structure for a while or using a powermeter for sometime you will know what you can for a 20 minute or 1 hour test within about 5 watts (10 max) in my experience. This knowledge will come from the workouts you are doing. You might get a nice surprise if you have increased your duration or intensity (or both) between the time of your last test.