Doing another brew today, a half batch. I measured the space between the bottom of the kettle and the bottom of the grain basket - 6.5L.
So my mash water now will be calculated similar to the Grainfather. (Grain bill in kg x 2.7) + 6.5. Just doughed in and the mash looks fairly spot on.
Also, an update on all the gravity readings I took previously in the thread. I just got a new hydrometer and never checked it in water. It's reading 3 points higher than it should be, so take 3 points away from all reading I mentioned above.