This is incorrect and will produce the wrong value for a number of different reasons. The 20 minute test suffers too much from anaerobic energy contribution (since its actually a supra-threshold effort) and the 95% correction factor is too variable based on individual physiology and training state.
The truth is that finding your LTHR is challenging and it usually requires multiple data points to make a good assessment.
The following types of tests will get you close:
- Take the average of the last 20 minutes of a longer steady state time trial (more than 30 minutes, 40 is better)
- Highest 60 minute average heart rate from a steady hard race (RPE 9 or 10)
- Ride at a well-established FTP until exhaustion. The second half will be close to your LTHR.