I ensure I'm properly hydrated in the evening then weigh myself in the morning after the AM pee. For me that seems to be the most consistent time of day. In the evening I'm all over the place depending on exercise and eating.

Unfortunately, I hit my minimum weight (i.e., poop) at about 10AM at work (LOL) but I'm not going to take the scale to work. I can't seem to train myself for a morning poop either.