Acetaldehyde is an organic compound that is produced during the conversion of sugar to ethanol. It's an intermediate phase of that conversion. It has a very distinct "green apple" flavor. If you have a significant quantity of it, that means you have incomplete fermentation. It can be caused by...
I also want to add that 20lbs is for average efficiency without sparging. If I don't sparge and just drain my mash, it's around 45% efficiency. You can always boost the gravity with table sugar (don't exceed 10% of the fermentable sugar in weight).
Also, keep in mind that with high alcohol...
I'm not 100% sure, but I think the reason people often quote you at percentages is because the efficiency of brewers and their setups can change. You need to gauge your efficiency and then use that to determine how much grain you need and then use the percentages to balance the flavor and...