If you want to check the result just do 2x20 at 100% and you will know. Alternatively there is a thread about if you can hold your FTP for an hour.
For what it’s worth I don’t consider myself a very good ‘high end’ cyclist (in fact my goals for 2020 are based around a 40k TT) but still prefer the ramp yet. The effort to complete 1x20 or 2x8 is a bit much and impacts my Thursday workout and you sort of have to do those formats guessing what your number is likely to be.