Roger,
I think the "offset" is the same as reset.
There may be a more elegant way to do it, but I go to the "Play" screen, scroll down on the right side until the alignment tool appears. Long press on that tool and a menu screen appears. Scroll down to "Roll and Pitch Offset" and press "Set". That is the only way I know to do it.
I use multiple levels when calibrating, and you have to decide what you want to be "level." The tripod head? The camera? The digital back? All these things can be slightly different. For example, the camera may not sit level on the camera plate, which may not be oriented level into the tripod head. The best way I've found to do this is to swing the camera around in 90 degree increments using a panning base. When the sensor is level, it should read the same in all four directions. Once I find that position, then I reset the alignment tool.
I think this tool is kind of useless because, at least on my back, it randomly goes out of alignment. What good is a level if you don't know if or when it is level?
Dave