It would be hard to determine how well you're doing as different practice papers have varying difficulties. Hence a 40/45 in one paper could be an amazing mark, but in another exam it could be an average mark. Based on what company's paper you completed, you can approximate how well you should be doing. For example, ACER's practice exams are considered easier than the actual UMAT, and so you should be getting high marks in them, while on the other hand, MedEntry's practice exams are thought to have sections which are harder in difficulty than the actual UMAT (especially in Exams 11-15), so what might seem like a low score in those exams could actually result in a high percentile.
To know if you're doing well, you could ask other people and compare their results to yours. If you're doing exams online and are provided with percentiles for each sections upon completing the exam, that indicates how well you're doing compared to everyone else who did the exam. However, be sure to realise that people who do online exams from a company are usually more dedicated to doing well in the UMAT, and hence they can skew your perception of what a good score is.
I don't think you can directly calculate the approximate total raw mark for a real UMAT exam from what you get in a practice exam, so your best bet would be to aim as high as you can. Examine the questions you incorrectly answered, and determine how difficult they are. If you're getting a lot of the easy questions in a specific section wrong, then that indicates that you might not be doing well in that particular section, and should actively try to improve.
