I think the key is to shoot at least 3 shot groups before making any adjustments to your scope. You should be able to get at least a 2 - 3 inch group even off the hood of a pickup. You shouldn't make any adjustments to your scope based on a single shot unless it is way off and you are really close and have a good rest.

When sighting in a rifle I usually start at 50 or even 25 yards until I get it pretty close and then I move out to 100 to fine tune it.