Sorry i conveniently inversed the calculation base on the previou poster's calculation for his understanding.
Anyway i've read Barista's handbook by Scott Rao too which is quite plain and easy to read. And i've read a few other books/studies backed by research.
As for coffee oil, there is a difference.
Oil mitigration starts starts during roasting and goes on during degassing outward through the cell pores. The condition of the bean is different.
Partly the reason, peaberries, maragogype and other large berry cherries are much appreciated
As for density, coffee solids play little influence, even adding with sugar raise density to around 1.08g/ml. TSD maybe 20mg-60mg per ml.
Anyway what previously mentioned, weight is a better count, not volume.
Crema should be persistent at just 10%, not necessary thicker the better.
As for percolation time, time is not an independent variable but rather the result of the other parameters.
Like i mentioned previously, grind, pressure, temp, tamp all play a part.
At higher temperature, it does bring along more flavours, but as well its bitter substance, which does affects the overal sensory appreciation as the bitterness will overwhelm certain delicate notes.
Thus there's no clear ruling going high is right. It's not about the solids, it's also about the lipids, volatile aroma, carbo, etc.
And espresso are highly not recommended to be extracted abv 92 degrees, unless a few conditions are met.
As for your definition of over extraction due to more extraction volume, i don't really agree because it's not considered that way. It won't burn the coffee as a real overextracted coffee do. By a more commonly known espresso, over extracted espresso may have white foam with big bubbles, if water temp was too high, a white spot can be found in the middle of the cup.
As for extraction standards, it depends on where you come from.
Preferably getting 25ml in 25-30 seconds in Southen Europe which includes Italy, Spain, Greece, etc.
In fact it was 6.5g +- 1.5g, 90 +-5 degree c, 30+- 5 secs.
There's no certain criterion but consunsus on a percolation time is in the optimal range of 25-35ml. Volume again depends on the preference.
Northern Europe like Germany Holland, Scandinivia and USA are the ones who may ask for up to 70-120ml.
Of course they will go for coarser grind so water pass more easily by different names, or lungo as you call it a taller espresso.
No one is a joke or getting ridiculous, a joke is probably when you are unable to appreciate by its form.
However short below 10 secs extraction, though most crema out first, is considered under extracted, tasting dull and diluted.
A lower dose 6.5g for 30ml, though almost 3 times more, doesn't mean that it will be bringing much less.
If one is to use 6.5g to achieve the same 30mk, the only way possible is he is grinding way finer than the guy using 20g to get his shot.
When it's finer, the exposed surface area increase exponetially allowing more extraction surface area and hydralic tractions.
Water don't go through the coffee fines while extracting, it passes by and bring along the oil, solids, and chemical.
And this is why a lower dose should tamp even lighter, and why people see Italians barely tamping at all.
As the powder are so fine, any hard tamp will cause channeling issues. They also use lower temp and higher pressure for same relating reasons. And like what Scott Rao had mentioned, the pressure during extraction dwarfs the pressure of tamping. To be honest, i'm not a big fan of Scott Rao, i only like it because it's easy to read for beginners. We used to let our part timers read it. There are better materials out there. I forgot which part i read, but i think he is not right in everythning.
Of course baristas even myself prefer to go for higher dose. Because the particles being bigger is easier to channel and easier to extract, but not really necessary nor i personally recommend. A single ligher dose is harder to extract than a heavy one, particularly with a single 7g basket.
End of the day, the most important chapter i learned is not the science. Is the psychology of perception.