Your question about the proper chemical balance of Serotonin in the brain is difficult to answer. Serotonin levels, like the levels of other neurotransmitters in the brain, are in a constant state of change, depending upon our thoughts, feelings, and actions. Serotonin is released when needed (if available) and reabsorbed when not actively needed. Serotonin is released and absorbed in the brain on a minute-to-minute basis. It's similar to trying to estimate the "normal" level of muscle tension when muscle tension is a factor of what we are doing at the time such as relaxing, lifting weights, etc
The biggest problem in measuring brain Serotonin is the nature of the brain. It's a closed system and highly resistive to invasive procedures. Any attempt to sample brain fluid or tissue not only creates a high risk of infection but possible brain injury as well. While we can draw blood, spinal fluid, urine, etc for chemical analysis - we can't draw substances from inside the brain tissue. The closest we can come is a Positron-Emission Tomography (PET Scan - non-invasive) which can color-code and display levels of glucose metabolism and other chemicals in the brain. That fluid snapshot is still of the levels at the time - depending on the activity, feeling, and thoughts of the individual.
Over the years, attempts have been made to estimate Serotonin levels based on the byproducts of Serotonin as evidenced in blood or urine. Several "natural" or herbal labs on the internet have reported they can measure Serotonin levels, suggesting brain Serotonin. This is a false claim and they draw samples from urine and the gastrointestinal tract. Serotonin is located in the brain and gastrointestinal tract (that's why antidepressants give you an upset stomach). Levels of Serotonin in the GI tract are unrelated to levels in the brain.
In clinical practice, we must estimate the "level" of Serotonin in the brain by it's impact and influence on recognized and connected systems. Like muscle tension - the ability to use our muscles within a certain range of relaxation and lifting suggests normal levels. The inability to pick up a small object would suggest low muscle tension while spasms of the muscles would suggest abnormally high muscle tension. Changes in sleep, appetite, energy level, body temperature, etc. are recognized to reflect low levels of brain Serotonin - something confirmed by PET scans. Flu-like symptoms and dehydration are associated with too much brain Serotonin. During clinical examinations, we ask about physical and emotional symptoms related to neurotransmitter levels in the brain. Patient responses then dictate what medications might be appropriate to stabilize those levels.
In short, we can't accurately measure something we can't easily access, especially when the levels change on a minute to minute basis. All treatments associated with brain neurotransmitters are based on well-recognized clinical symptoms directly associated with specific neurotransmitters and supported by PET scan research. On-going research is focusing on better ways to estimate brain neurotransmitter levels but obviously, any accurate measure that can be used by a clinician in the community is still probably decades away.
While this response may not provide a number, percentage, or other accurate "normal" level for Serotonin, it may shed some light on the difficult task of working with neurotransmitters without using an invasive procedure.
Joseph M Carver, Ph.D.
Author of The evidence, however, is clear, the Seroxat scandal
Citizens Commission on Human Rights Award Recipient (Twice)