Did some googling, and these two threads seem to indicate that the game calculates tech level for later starts using income, tech group, average year of the techs, misc modifiers (NI:s etc).
I dind't find anything concrete or specific about these calculations unfortunately, but that may be due to low googling skills on my part.
Is it possible to use some approximation of income (we've got/will have income calculated, right?) and the tech cost formula on the wiki? If that's not feasible I guess we could just approximate it using average dates for the techs and scaling it down for non-latin tech groups. Unless someone else has a better/more elegant idea, of course.
I probably should have started with googling. However, I've been spending my morning crunching numbers, and I have some initial conclusions. I'll write up a post on what I have soon, as I'm soon to busy for the rest of the day.