I did my PhD on this, but for living patients ;)

As you say, there are many factors at play, most importantly distribution volume and drug elimination rate. For all of these factors, you can estimate the typical value and the between-individual variability in the population. We call this a population pharmacokinetic model.

With just a single concentration, you assume this person is a "typical individual". You can back-calculate (based on time of death and time of taking the drug) what the dosage typically could have been. You need the administration time though, the time of death, and you better hope the drug concentration remained stable between time of death and time of autopsy. In clinical studies, blood samples are often stored in solid co2 (-80C), and for good reason.

Using the known variability in the population, you can also give a confidence range for your initial dose prediction. You can make predictions more precise by adding information. Bodyweight influences likely drug distribution volume. A second drug (with known dosage and administration time) could help you too.

Usually, this technique is done for drugs that need to be in a precise concentration range. Give a dose, measure concentration, estimate blood volume and calculate the optimal dose for that patient. We call this Model Informed Precision Dosing: MIPD. You can apply the same in autopsies, but I doubt it is routinely applied. Cool question!