This recipe comes from Martin Keen and Homebrew Academy. I found it while looking for a slightly different take on a London Brown Ale rather than brewing my usual recipe.
The recipe did not specify mash efficiency, target boil starting volume, or post-boil volume (it may be in the video which I have not watched) but the overall grain weight wasn't too far off from my usual so I just went with it. I will likely tweak it some, but with the high efficiency I tend to have on small beers like this, I'm not too far off of my intended original gravity.
To account for the use of S-04 rather than lower attenuating yeasts I have used in the past for this style (mostly out of convenience) I mashed a bit higher at 155-156F on the first attempt.