Back-gouging depth

The backgouging depth is what you need to remove at the root of the first pass to get all defects like cracks or poor penetration out cleanly. If you're too timid and leave some in, you'll have trouble with the weld later. But if you go too deep, you weaken the material and alter the structure unnecessarily. So: measure right and remove just enough so the bottom is really clean, but no more.