If you want to make your life easier by going between positive and negative stroke-dashoffset values, think again. Safari will not allow it and will promptly explode.
A quick way to understand how stroke-dashoffset values are calculated is by a mix of voodoo magic and STD’s. The % values seem pretty random, but pixel values are based on the length of the current stroke. I would recommend using pixel values because they are easiest to visualize.