How to hit gravity:
The core principle is to match up the percentage used of each grist component (malt or adjunct) with the extract in litre degrees obtained from each component. From these, and the known volume you’re brewing, you can work out quantitatively how much of each malt you will need to theoretically hit target gravity (given a theoretical 100% efficient extraction).
At this point you need to know the efficiency of your brewing system witch is derived from the difference in what extract you should get from the malts your putting (as just described) in comparison to the extract you actually get in the fermentation vessel. Additionally what you can do is get a gravity reading of wort from the copper (once its full) termed the copper up gravity. Under the same logic you can use this to get a gauge of the efficiency of your mash separation method (whether mash tun, lauter tun or mash filter), and what the gravity of the wort will be in the fermentation vessel. I have often contemplated using this method, but find it takes a little long to bring the temperature of the wort sample down when I have only minutes to return it to the just starting to boil copper.
That’s the basics, some other things to consider are the various factors that effect extracts, including:
Length of the copper boil; boiling longer drives off vapors condensing the worts sugar content. Very simple. In fact some strong ales such as Bass No1 barley wine rely of extreme boil lengths to maximize start gravities.
Malt quality; for example over or under modified malts can give poor extracts that can be harder to ferment due to insufficient wort nitrogen, watch out for them!
The mash separation system used and how the grain is prepared; For example the use of a mash filter over a mash tun gets a higher extract into the copper using a more finely ground grist. Mash tuns require coarsely ground malt that allows the malts husk material to act as a natural filter bed during run off, using a finer grist increases extract but the mash tun user risks losing the natural filtration from the in tact husk material. Secondly the grain is also sensitive to sparging temperature, the hotter the sparge the higher the extract. I tend to sparge around 76-77oC, much higher and you can risk extracting unwanted bitter tannins from the grain.
Mash and wort acidification; Increases extract and run off rates, reduces color formation during the boil but does lower hop alpha-acid isomerization meaning you get less bitterness from the hops used (but that’s a different subject). Mash and wort pH can be tinkered with via various water treatments, the use of acid malts (often used in pilsner and pale lager formulations) and decoction mashing.
A final and very valuable factor is volume; Stopping the run off from the mash tun early leaves you with lesser volume of wort at a higher gravity than if the total volume was collected. Some brewers do this on purpose to then liquor back a controlled volume and hit target gravity, a method called high gravity brewing.
However taking these factors into consideration I have no clear explanation for the reading. The only logical one is that the base malt used was from a stray bag of marris otter at work that was delivered by accident and no malt spec was available for it, so I had to guess at the extract alittle.