Add a known amount of water, and measure exactly how much water you are able to drain out of the system. The amount you can't drain is effectively your dead space. Mine is ~ 1/2 gallon, slightly less. Take that into consideration for your water measurements.
Why would you have dead space in the brew kettle? Wouldn't you just pick up the kettle and pour everything out? Of course there would be the grub issue. I will try the recommendation for both my tun and kettle and see what is left. Thanks guys.
Not everyone picks up the kettle and dumps it out.I would venture to guess that most people don't actually. I have a spigot on my kettle that drains through a hop stopper screen.