If I were Bernal I'd want more than 2 minutes on Evenepoel. in the final TT to allow for any technical issues and just to feel comfortable. Each KM the greater potential for loss for someone like Evenepoel versus a TT'er like Bernal.
The base calculation is as follows:
30.3/8.6 = 3.52
3.52 *20 = 70.4 seconds = 1:10
However, you'd need to use this type of calculator
Choose temperature, altitude, tires, CdA, drivetrain efficiency and calculate the speed for a given power output.
to address drag coefficient, CDA, altitude, etc to determine how that time would be modified.
On top of which is a bonus to Bernal for wearing Maglia Rosa and going last. Granted this would need to be researched to determine the expected increase granted by being the leader
Of course again one would have to measure "decay" of the riders throughout the race.
In March Bernal lost 0:55 seconds to Wout in a 10.1km ITT 0:44 to Ganna and 0:43 to Pogacar
In February he lost 1:09 to Ganna in a 10.74km ITT with final slope.
His most recent ITT results seem to suggest that he's at a severe disadvantage in an ITT.
PS my initial times were wrong. I would put Evenepoel 33:10-33:25 and Bernal around 35 - 35:15 for the ITT on stage 21.