There are plenty of deficiency guides out there.
JamesC's Plant Deficiencies sums things up pretty good. As you correctly point out, it's very easy to confuse things between different plants. There are some fairly obvious cases such as with N deficiency, everyone can identify yellowing, so that's fairly easy but other's are not that obvious. Sometimes, depending on ambient conditions one deficiency can look like another especially if the hobbyist ignores the other signs in the tank. I see a lot of posts where people see deformed or crinkled leaves and assume it to be a Calcium or Magnesium deficiency when in fact it's a CO2 deficiency. Another variable that few think about is that there can be a difference in behaviour between a chronic deficiency (long term) versus an acute deficiency (sudden).
The reason one needs to specify which leaves are affected is due to the relative mobility of nutrients within the plants. Some elements such as NPK are highly mobile and there are many chemical pathways for them to move from leaf to leaf. In these cases the plant can move the elements from mature leaves to feed critical new growth. As a result, the mature leaves suffer as nutrients are withdrawn from them in the same way autumn leaves suffer when a tree withdraws it's nutrients for the coming winter. Other elements have fewer chemical pathways to move about so they are relatively immobile and the plant is not capable of transferring them to the new leaf. If a shortage occurs with that particular element then the new leaves suffer the most. So for example, both an N shortage and an Fe shortage cause yellowing but since N is highly mobile there is a higher probability that mature leaves will suffer as the plant redistributes N to care for the new leaves, whereas since Fe is less mobile it is more likely to stay put in the mature leaf and therefore new leaves will be lacking in Fe and thus will tend to show discolouration. Micronutrients tend to have much lower mobility than macronutrients but some have better mobility than others.
Another complication is that some species have multiple or redundant reaction pathways. What this means is that if a primary element in a certain chemical reaction is in short supply then the plant can substitute a different element in a similar reaction, so for example there are some chemical reactions that require K but the plant may be able to substitute Mg in that particular reaction. To complicate the situation even more, a shortage of one element can be exacerbated by an over-abundance of another. These are some of the issues when people try to control their dosing by using ratios, or when an attempt is made to micromanage concentration levels. It's a real minefield.
Phosphorous is a highly reactive element. In fact, it's so reactive that all the P that has been created in the universe has already reacted with something else so in nature you will never find P by itself. A lot of energy is released when P bonds to something else and it takes a lot of energy to break a P bond. All creatures great and small therefore use P. If you conceptualize the balance between energy consumption (feeding) and energy dissipation (working) in living things as an economy, then PO4 is the cash currency of that economy as a direct result of it's chemical energy potential. Within the plant P shortages have system wide consequences which effectively and indirectly can be exhibited as slow growth. This is illustrated by considering the end product of photosynthesis - the food reserves which are created is a Phosphate sugar called "triose phosphate" (AKA Glyceraldehyde 3-phosphate, or 3GP). Chronic low P levels have the inevitable effect of low food production, therefore, following the energy currency analogy, I consider low PO4 levels to be the equivalent of a Third World economy. It always amazes me when people panic regarding perceived high PO4 levels in their tap water, or when they use PO4 removers in their filtration. Three months later these same people are asking questions such as "What can be done to improve plant growth and reduce algae?"....
Cheers,