Thanks for this info, I'll try some other tactics on bigger beers ( I don't make many) and if not just do the partigyle bigger volume and spread it over 2 days.How good are partigyles... Always worthwhile if you have a bucket handy to make starter beers and yeast cakes.
If you ever forget to measure preboil gravities, you can calculate the difference using the known volumes and post boil gravity.
Preboil Volume x Preboil Gravity = Post boil Volume x Post boil Gravity
Preboil Gravity = Postboil Volume x Postboil Gravity / Preboil Volume. Note it needs to be the volume in the same vessel, like the kettle. So assuming you got the anticipated 26L in the kettle... You would have had a preboil gravity of ~1.075 (24 * 81 / 26)
I always find that when you push your equipment past the sweet spot, the efficiency tends to suffer. That said Efficiency will drift a few percent between brews even when inside the sweet spot.
Best thing about electric kettles is the evaporation rate is predictable and consistent. These days I undershoot my efficiency in recipe calculators (set it ~5-10% lower than my average). So I am able to predict and hit my target gravity every brew using the equation above, paired with known evaporation and dilution. Simply letting the volume vary with efficiency (usually get a bit more volume) and locking in the gravity I want to achieve.
I'm not sure why the brewers friend has such a big change, but I wouldn't worry about it. If your beer is good, all is good.
I have a clone recipe for Harveys Elizabethan barley wine and could just parti gyle the rest to make some of their best bitter ( if I could get their yeast ) .