In the old days we just wrote down when we killed them and then just checked back, one guild I was in used to phone round if the mobs were up so that we would be the first at them.. Obligatory raiding Fear at 3am was fun, but then it wasn't. But if you dont know when they went down, then you don't know when to start the timer. The respawn on most of the PoP gods were 3 days +/- 12 hours.
Sorry to be negative. It was a good idea.