Tom Paris' Warp 9.9 = 4 billion mps is what I use to calibrate what Warp 9.9 is relative to my improved WF 3.0 scale.
That's the best on-screen canonical statement we have.
WF 3.0 scale is just WF 2.0, but remove the hand drawn curve after Warp 9 and let the formula run until infinity.
The #'s are still clear & manageable, plus you can easily calculate travel time between point A->B easily.
No Starfleet ship was actually seen achieving or sustaining Warp 9.9 for any appreciable amount of time under regular circumstances during TNG run (Enterprise D topped out at Warp 9.6 for sustainable speed)... except the USS Prometheus which was seen in the 4th season of Voyager (the Prometheus EMH mentioned that the ship was en route to Romulan space at Warp 9.9 - and last I recall, that ship wasn't shaking or straining itself to maintain that speed - Voyager on the other hand was threatened with an imminent structural collapse by just approaching Warp 9.9 in 'Threshold' episode which forced Chakotay to DROP the speed to Warp 9.5 - and we have at least 1 other instance where Paris actively said to Janeway that he can try maintaining Warp 9.75 (not 9.975) for 12 hours - when they decided to go through 'The Swarm' territory.
This is why I think the 9.975 figure (which was never attained in practice) was incorrect and what the writers probably wanted to say was 9.75 for a limited period (albeit to be fair, for the Federation in the late 24th century, it should have been more than possible to attain/maintain Warp 9.975 indefinitely as a sort of base cruising speed, with say Warp 9.9999 being 'maximum sustainable cruising velocity' which can also be maintained for a very long time - or enough to get you to Andromeda Galaxy in about 4 hours - so, that speed would be used to reach other galaxies and in emergencies obviously, whereas lower speeds of 9.975 for example to explore the galaxies in a relatively small/manageable amount of time).
So, if Voyager's (actual) 'temporarily sustainable cruise velocity' was say Warp 9.75 for about 12 hours... Warp 9.9 (maintained indefinitely) on USS Prometheus seems reasonable only 4 years after Voyager's launch.
And if that progression (of increasing Warp factor by 0.15 increments every 4 years is maintained, then in 20 years AFTER the USS Prometheus (or, by the time of Star Trek Picard), sustainable and cruising Warp speed would technically be Warp 9.975 (3.435 million times C).
But let's assume that the closer you get to the TW threshold, the more difficult it becomes to achieve high Warp velocities due to energy/fuel consumption also increasing by large amounts (exponentially, just like the speeds)... which would necessitate developments/breakthroughs in energy and fuel efficiency (that said, a starship could technically/probably produce/emit Soliton Waves to increase its energy efficiency to similar levels like they saw in the original Soliton Wave experiment... 400% increase in efficiency - or at least REALLY close to that).
100% increase in efficiency would be double of what the Enteprise-D had.
400% increase in efficiency would be 5x better than what the ENT-D had (which at the time of the soliton wave experiment and TNG run could maintain Warp 9.6).
So, that (with the Soliton Wave efficiency trick) would probably allow for a sustainable Warp velocity of about Warp 9.925 on the USS Prometheus.
Warp 9.91= 42,946 times speed of light (double increase in speed and energy consumption vs Warp 9.9)
Warp 9.92 = 85,892 times speed of light (another double increase in energy consumption vs Warp 9.91 - or quadruple increase in speed and energy consumption over Warp 9.9)
Warp 9.92 would take a ship about 31.5 years to reach Andromeda galaxy.
Still quite slow... but in contrast, it would need under 2 years (approximately) to go from one end of the Milky Way to another (if you acknowledge revision to the Milky Way measuring 150 000 Ly's from one end to the other ).
By the time of Star Trek Picard, I think SF ships would be capable of those and greater speeds (aka, Warp 9.975 - or about 286 days to reach Andromeda galaxy).
Though, this is still trumped by Quantum Slipstream V2 (which the Voyager crew designed with Borg technology and Benamite Crystals) that topped out at about 10 000 Ly's per minute (rough translation speed is Warp 9.9999)
Quantum Slipstream V1 (300 Ly's per hour) would be just under Warp 9.975.