Unfortunately, there is a lot of misinformation on the internet and on forums about how UMAT scores are calculated.
Some people and companies claim that your UMAT score or percentile ranking can be calculated from your raw score (the number of questions you got right). This is naïve, simplistic and demonstrates ignorance. Not to mention it is irresponsible to provide students with incorrect information.
ACER uses a complex computer program to calculate UMAT scores. Even if you buy it (it costs a few thousand dollars), you cannot replicate the results since the assumptions ACER uses to run the program is kept confidential.
The way your scores are calculated is very technical. ACER uses the Item Response Theory model which takes into account parameters such as the difficulty of the question, question discrimination (point biserial correlation coefficient) and guessing. For example, this means it is impossible for anyone to get a high score through pure random guessing.
Further details of how UMAT scores are calculated are discussed in MedEntry UMAT workshops.