First of all, the leaflet in question also states:
Your doctor will tell you how much Daktarin
oral gel to take and for how long you should
It is essential that your therapy is supervised by a physician, primarily because infections of the stomach and the gut may be serious. The doctor can monitor how you respond to therapy in terms of both efficacy and safety (i.e. possible side effects).
That being said, it is important that you understand the information provided in the leaflet, so here goes the explanation.
20 mg is the dose of the active substance. This is a common practice because there may be preparations of different concentrations.
There is an example of dose calculation based on body weight in: Pharmacy Technician Certification Review and Practice Exam
edited by Barbara Lacher.
The other problem with you calculating the dose yourself is that the concentration is expressed as mass fraction, from which one can calculate the total mass of the preparation to be taken. On the other hand the measuring spoon provided (at least according to the attached leaflet) is scaled in milliliters and measures volume. To convert mass to volume you need the density of the preparation, which is not given in this document. This bit is probably known to your physician/health care team.