Human equivalent dose, concentration, volume?

They are only connected in that  both are mathematical ratios, which are most easily understood as fractions. (all ratios are fractions but not all fractions are ratios). Milligram is a unit of mass and milliliter is a unit of volume. This is how much drug is in a given volume of solution. (e.g. we might say there is 2 mg of drug for every milliliter of solution, where the word "for" is notated as a fraction bar, giving the fraction 2/1 on drug vials the 1 is usually understood, so we say 2 mg/mL) Likewise, milligram is the mass of the drug and kilogram is a larger unit of mass that corresponds to body weight of the patient. (we are measuring an arbitrary amount of drug for each unit of body weight of the patient)  Thus, when we say something like  2 mg/ kg of body weight and the patient has a total mass of 54 kg. (a value that will differ from patient to patient)  the appropriate dose for that patient is 118 mg - we then convert how many milliliters we need to correspond to 118mg of drug. In the given example Since we know that every milliliter has 2 mg drug, we then know, by mathematical formula, that we need 54 mL of solution to make 118 mg of drug. We can solve this by creating equal fractions - if a/b = c/d then a(d) = b(c). In this type of  calculation, the different units are not relevant to the math itself because we are solving for proportion, though we are properly using two different equations to come to the final answer. The answer to the first equation is one of the known variables in the second equation- it is then only solving for X using an algebraic expression.