Low light negatives will display increased contrast precisely for the reasons you said. So when you underdevelop the neg to compensate, you lose even more film speed, and slip further into reciprocity territory, and gain even more contrast. It's not just a single variable problem (i.e. exposure time), because the variable of development time is a factor as well.

If you want an easy answer to your question, just leverage the amazing amount of work that Phil Davis did last year and buy a palm pilot and put the BTZS expodev program on it. His film development curve data handles many different films and developer combinations. What is interesting is that not all films behave the same in different developers. The real champ from the huge amount of research he did was HP-5/Ilford DDX. It loses less speed (and consequently gains less contrast) than just about any other conventional film combo. But I can attest from personal use of this program that it works, and works extremely well. Because I shoot 12x20 film, I don't bracket, and his little program lets me do it with confidence. Never had a negative that wasn't pretty close to just right.

Clay