The way that I understand Tong Ren Therapy is it is a synthesis of western anatomy/physiology and eastern energy/acupuncture. So to find the best points required you need to understand and combine both systems. Sure you can just look up the points in the book, but that is just the beginning of the learning process.
For western, you look at points related to anatomy (bones muscles flesh etc.), circulatory, nervous, metabolism, endocrine, immune. This requires understanding of the systems involved. You are encouraged to get a diagnosis from your doctor so that you can treat the western aspects properly. Once you have a diagnosis, you can better choose the points that are related to the physical aspects of the condition.
A little internet research and thought can also help to understand how the systems work and help you to understand how the points in the book work. For example the vagus nerve is critical for anything that involves the internal organs because it is how the brain and organs communicate, so it is used often for any issue with the internal organs. Another example is the medulla that regulates breath and blood vessels, so that area may be involved with blood pressure or also with breathing control issues. Then you can move to the spine which has nerves that also map to parts of the body. And for complaints in specific areas of the body, tapping on the blood vessels that go to that area to improve circulation can help. And then you can also use the Ouch Point which is the location of the pain is felt or where the disease has manifested.
For eastern, you look at the meridians based on location and energy function that are related to the symptoms. This requires an understanding of the energy pathways. For a basic understanding you can just look at the doll and get some idea of what parts of the body are affected, but there are also points that are more subtle and require deeper understanding. There is also the huatuojiaji points of the spine which reflex to areas of the body, which is beautiful because it is so simple and intuitive once you see a map of the nerves that run from the spine out to the body. Sometimes being sensitive to energy can help deal with the energy blockages, but sometimes that can also distract you if you start playing whack-a-mole and chase the energy effects you feel without understanding how the meridians function or the nature of how energy moves around. Sometimes less is more and it is better to focus deliberately on specific points that are the main cause of the blockages rather than jumping around a lot in reaction.
As an example of combining the two systems, say you have a headache. If you know that it is related to muscle stress, etc, then you would use points like the SCM to relax the shoulders. And in terms of energy, for a headache you often want to tap on the LV3 and K1 points to bring the energy down. Note also in this example that the areas you tap are not the same as the area of the pain symptoms. But it is more related to the root cause of the problem.
When you do Tong Ren, the idea is to understand the root cause of your energy blockage. In general you want to start with the most serious and oldest condition(s) which is the origin of your issues. Then you move out to less severe and more recent issues. And I think that it is also a very important comment above that mentioned looking for points in common. You can sort of do this by just taking your complaints and listing out all of the points for each and noting where there is commonality. But by studying further you can take this approach to a much deeper level and actually focus in on the blockages by understanding the systems involved. Becoming an active member (tapper) in a Guinea pig class is a great help here. It takes a while for the information how it all works to seep in.