"... each of us responds differently to the same level of the drug at different times and under different conditions such as when, what and how much we have eaten, as well as activity and levels of stress, so knowing our l-dopa levels at a given time isn't likely to be of much help.
Finding and keeping a consistent routine of sleep, activity, eating and taking medications works best for me. ..."

PD is both extremely complex and very individualized. We have different requirements at different times of the day and under different conditions and we each differ from one another in our own particulars. A consistent routine serves as a baseline and, from there, trial and error allows us to derive something that works. At least until something perturbs the baseline. A bad night's sleep. Spousal conflict. New medication.

A log of some sort is invaluable, especially at first. Yesterday affects today. When you go to bed and when you get up is more important than how much sleep you get. At least for me. What we eat is important but when is even more so. At least for me. Giving us a handful of pills and telling us to take three a day is pretty poor care.

Another reason for going to all that trouble is that it let's us reclaim some of our power. Losing control of our lives s a terrible blow. As you say, many of us are accustomed to the opposite way. Suddenly, we seem to be helpless. We are not.

A good book is "PD: Reducing Symptoms with Nutrition and Drugs" by Dr. Geoffrey Leader and Lucille Leader. Some of the chapters deal with nutrition and drug response, nutritional therapy for PD, dopamine precursors, cellular energy, blood sugar, how to figure out your own individualized L-dopa schedule, reducing dyskinesia, liver care, food intolerance, food additives, and that's just the first 14 of 36 chapters! And it is written for patients.

We are clever monkeys. Now, if only we can find the right broken stone....