At the end of Solo: A Star Wars Story when Han and Chewie are talking about the Kessel Run, Han says:
not if you round down.
What is he rounding down from? I guess it's not 12.5-12.9 as it seems Han is being a bit liberal with his counting.
In the Solo film's official novelisation, L3 notes that the run was completed in "a little over twelve parsecs". At this point in the film she's become integrated into the Falcon's primary memory core so we can assume her opinion is solid.
And then the Millennium Falcon had made the Kessel Run in a little over twelve parsecs.
In the Solo film's junior novel we actually have Chewie's dialogue translated for us. He feels that it was closer to "thirteen" parsecs.
“See that ship?” he said. “She just made the Kessel Run in twelve parsecs.”
The worker paused momentarily, stone-faced and clearly skeptical, and Chewie growled: “Actually, it was more like thirteen.”
“Not if you round down,” Han said, and walked over to Lando, who was looking up at his baby with visible anguish. The ship looked nothing like it had when they’d left Vandor. Running the gauntlet had ripped off all her precious added adornment, leaving a stripped-down, mean-looking freighter covered in dents and scrapes.
This inconsistency may be due to Chewie rounding up rather than down which suggests that it was probably close to the halfway mark between twelve and thirteen. There may also be some inconsistency about what represents the start and finish of the 'run' given that the ship looped around a black hole.
In the movie, it's not specified just how many parsecs they traveled in the run, so the line simply indicates that, while he did decrease the distance traveled (according to Star Wars: The Essential Atlas, the Kessel Run was an 18-parsec trip), it was not actually as low as twelve, although he's stating it as such. Presumably, it's not high enough to reach another whole number.
Notably, by the time of Episode IV, he's claiming that he "made the Kessel Run in less than twelve parsecs", indicating that his exaggeration has grown.
A parsec is a unit of distance, not time. Solo was not referring directly to his ship's speed when he made this claim. Instead, he was referring to the shorter route he was able to travel by skirting the nearby Maw black hole cluster, thus making the run in under the standard distance. By moving closer to the black holes, Solo managed to cut the distance down to about 11.5 parsecs.
This says he made the Kessel run in about 11.5 parsecs, which probably means he did it in 12.1 parsecs to 12.4 parsecs. This is the closest reference about how long it took him, so we can assume that he did the Kessel Run in about 12.3, or so parsecs.