The key to zeroing in a scope is to have everything the same each shot. Something like a lead sled is best, (to me) but even that must remain in the same location as the 1st shot or it will be off a bit. If your using a portable table even that need to stay the same through all the shots.
So if your going to sand bag, which is more than fine do the following:
Mark where the stock is on the front (how far forward/back) and (if you use it) the rear is sitting on the bag. Long vise type bags are good as the rifle rests in the center of the bag so it's easier to measure.
Measure the height of the barrel tip and the butt end of the stock from the table while the cross hairs are lined up.
You can tie a string to the barrel in front and cut it off right at the table to save you from measuring each time, same with the butt and just tie it to the sling hole. Use a couple split shots if you like. This will also help a bit in making sure your perfectly verticle (depending on how anal you want to get :-D) Speaking of which its very important to have your scope leveled properly during mounting (another reason a leadslead or vice is handy)
Then the hard part is how hard you keep it against your shoulder. When I do this I tend to just let it touch me enough where it doesnt move the scope sight off the setting.
It is a pain in the butt to do it this way but you'll get the best result from what the rifle can do if as many varibles are the same with each following shot. (which is why a lead sled is so useful as it sits the same everytime as long as you draw a circle around your footing cups and place them in the same spot each time.)
All that said, if ya just sandbag it and more or less follow all that but not to the extreme, your more of less get it sighted in, but you'll never know exactly how accurate your rifle is.
