A good benchmark is that if you can hit 19:30, your FTP has stayed the same. If you can get to 20:00, you’ve definitely added a few watts. So find that classic, find the bit where the hands are in the air, skip back 20 minutes, and press play at the exact time you start the test. If you’re still going when Sagan wins, you’ve won too.
More great tips in this thread.