Just remember that the bearing and distance provided by the unit is straight line, which may not be the way you will actually travel. This is insignificant if you are on a trail, but may be critical if bushwacking. The GPS disregards the 400 foot deep slot canyon in the middle of the one half mile to your way point.

Depending on the contour interval, this slot canyon (or similar barrier) may not show up on your map, so you need to acquire a feel for the geography of the area in which you re traveling.

To get back to your original question, I never program it in, because I never know for sure just how far I will hike on any given day. I start in the morning and stop when it isn't fun anymore (or the next feasible campsite after that point).