My recipe says to braise beef in the oven at 180 C/350 F for 1.5 hours. Dinner is in about four hours. Can I also get away with sticking the dish into the oven at a lower temperature (say 150 C/300 F or 120 C/250 F) and expect it to be just done for dinner? If so, to which temperature should I set my oven? Is there any way I can convert or calculate this myself?
This answer may be too late for you, but in general a braise can be done at a lower temperature as long as the food doesn't stay in the "danger zone" for too long. See this answer for more information on the danger zone.
For example, this recipe advise to have the oven at 300 F for an and hour per pound. In the tips section, it states that the author sometimes drops the temp to 250 F.
Braising is fairly forgiving and is typically a method used on tougher cuts of meat, so going slightly longer shouldn't be a problem.