Couple of things.
First, mash tun dead space is the unrecoverable wort which cannot be drained from your mash tun. This differs from the recoverable mash tun dead space in that this is the volume of wort which is not in direct contact with the grains but will be drained from the mash tun.
Second, your grainfather is an all-in-one unit so it really does not have a mash tun. Your losses at the end should be considered under "trub loss" since this volume not only is not lost pre-boil, but also participates in the boiling process and therefore ends up with a higher concentration of sugar than it would have at the end of the mash.