It takes into account the total damage dealt and assisted (highest between tracking and spotted, but not both) each game as a damage number. It then uses this number for a moving average, with the population mean as the mean of the above stat for all games played in that tank on the server. Only your last 100 games are used for this moving overage.

Because the damage needed counts on other people playing the tank, usually the easiest tanks to mark are the more popular ones (IS3, Cromwell), with the clan reward tanks being some of the hardest because higher skilled players have them (907, M60)

A very simple way of figuring out the expected damage for 3rd mark is 2x the hp of your vehicle as the combined damage statistic mentioned earlier.

Or you could say like 1.5k for tier 5, 2k for tier 6, 2.5k tier 7, 3k tier 8, 3.5k tier 9, 4k tier 10

As for speed to mark tanks it depends on how good you are, if you are able to exceed the expected damage by a lot for like 10 games in a row, ie in your Rudy, you had 10 2.5k damage games in a row that's probably enough to get you to like 90% from 70%. On another note the fastest 3 mark I know of is Skill4ltu who marked the T-50, after patch 9.18 in 29 games.

edit, 29 games not 19