Except for the time until you research your first tech, the tech cost only changes when you start researching a new tech after finishing the research on the old tech. That is, if you change research targets as a result of acquiring your current research target some other way (pod, gift, trade, theft, etc), your tech cost will remain the same, despite having more techs. Thus it is a good idea to delay new tech acquisition from just before you are going to finish research til just after you start out fresh on a new one (unless you are considering acquiring the tech you are about to finish researching, of course). Two or three techs can make a very substantial difference in your tech cost, expecially noticable in the early game, so it is well worth worrying about.
During that interval before your first research ship comes in, the game may bump up (or lower) your tech cost without your having to finish a research project first. This can be a very substantial downside to an early encounter with Zak or a run of luck at pod popping, say if you were just about to finish Cent Ecology, but now must wait 5 or 10 more turns to get it.
It can be a very sly strategem to trade or even give away a bunch of techs to an opponent in an MP game just prior to when they finish their research; they will be thanking you very gratefully while you ratchet up their tech costs.
BTW, I think that there is something wrong with that formula, unless one of the elements that were omitted in this "simplification" serves to keep costs lower at the very beginning of the game (like not counting the initial tech). According to this formula, your tech cost would roughly be about 30 or 31 with 1 tech assuming transcend/standard map (11+20) and 60 or so ((11+20)*2) with 2 techs and that seems to me to be way off compared to actual tech costs - by a factor of 2 at least, maybe 4 even.