Based on figures from the Siebel Institute of Technology, I've assumed for years that each pound of dry malt in a mash retains about 0.185 gallons of water. After taking a handful of measurements at home and at work, I concluded that the actual value is closer to 0.155 gal/lb (to Siebel's credit, my old assumption was based on their value for spent grain water retention plus a fudge factor-y variable of theirs that I accounted for in the same place). The obvious use for this variable is to help plan how much water you'll need for brewdays, but it affects other parts of the brewing process as well. For example, the gravity and volume of each runoff in a batch sparge is a direct function of how much wort is reatained by the grain. Because I use batch sparge simulations to generate my brewhouse efficiency table, its accuracy depends on spent grain wort retention. The values in the table have been updated accordingly.
For water planning purposes, it's important to differentiate between lauter tun deadspaces that contain mash and deadspaces that contain wort. In a lauter tun with a pipe-like manifold, the volume below the manifold contains mash. Because a mash will compact if you keep lautering after adding all of the sparge water, you can assume that all of your wort (aside from what the grain retains) will be squeezed out of the deadspace. In other words, these types of losses are included in the 0.155 gal/lb number. The space underneath a false bottom, on the other hand, contains mostly wort. Even if your lauter tun drains from the bottom, the grainbed will try to hold the wort beneath the false bottom like a finger covering a straw. If any wort manages to escape, it'll be grainy and shouldn't be lautered anyway. As such, the volume beneath a false bottom should be treated as a deadspace regardless of your vessel's outlet location(s). These deadspaces represent equipment losses, and I assume that wort lost to equipment geometry is equal to the total volume of deadspaces that contain primarily wort. To calculate total wort loss for a given mash, I add the equipment loss to the volume of wort retained by the spent grain.