So to use your example. Someone shared a recipe they have made. That recipe has 5G - 12lb Grain - And a target OG. If I recreate that recipe as detailed, my BHE would be exactly the same would it not.

It might not be the same, unless you hit both OG and fermenter volume the same as the original brewer. Mash extraction efficiency is the first part of BHE. The second part is the losses that are unique to your equipment and methods. BHE is then "Mash Efficiency" multiplied by "Fermenter Volume Efficiency" expressed as a percentage.

The first part is fairly straightforward and it seems you understand it perfectly well. It's the second part that is not making sense to you.

If batch volume is measured in the kettle, post boil, then mash efficiency is the same as brewhouse efficiency. There are no losses to consider, so the volume efficiency is 100%. But if you measure batch volume in the fermenter and if your equipment holds 0.5 gallons of trub you consistently leave behind, you now have a volume loss to consider, which will lower BHE. Trub loss of 0.5 gallons for 5 gallons means you're harvesting 90% to the fermenter. With that loss and a mash efficiency of 75%, results in a BHE of 67.5%. In other words, 67.5% of all the available sugar will end up in the fermenter. The final fermenter tally would be 4.5 gallons at 1.068.

So how would I know how to scale the grain bill accordingly? Each brewhouse obviously has different grain absorption rates, boil off rates etc. But AFAIKT none of that is taken into account with BHE which is why I was asking the question.

Suppose the recipe you purloined from the club meeting is based on the above BHE, but your BIAB only gets 65% mash efficiency, and holds 0.75 gallons of kettle trub. Add to that, you want 5.5 gallons in the fermenter to account for yeast and racking loss in order to get a full 5 gallons into your keg. Obviously, you will need 6.25 gallons, post boil. If you follow the recipe exactly, you'll end up with 5.5 gallons at about 1.050 to ferment. Your BHE with the change in extraction, volume and losses is 57.2%. Here's where the scaling needs to be with BHE, not just mash efficiency because to get the correct gravity AND volume, the ONLY place it can come from is

__increased__ mash efficiency. As you pointed out, your efficiency is somewhat fixed and predictable, so that's not likely to happen. Therefore, covering losses as well will give you the correctly scaled grain weight to get the intended OG 1.068.