This is frustrading. How do skill times increase? At first glance they looked like doubling each time. But now I calculated back and get an average increase of only ~25-30%.
Can you calculate backwards? You would only need to know the time spent on the current run. So if you were 13 days in and need 2.5 days to reach 60, that would give 15% average increase in time, with starting lvl 50 for Artistry.
Could it be something like this? Time for Next Level = Time for Current Level * (2 - X), where X increases with lvl.
Or just a bogus table with each value written in some arbitrary way?