The most important (only) measure of Study Mode success is "aptitude". I thought it was closely related to "percentage answered correctly". It is not. I have a scenario where I am answering 90+% of the Technician questions correctly, yet my aptitude is only 50% of the seen value. When asked for a definition, in part I got this "But we don't consider answering it correctly once to be necessarily indicative that you know the question". I find this quite bizarre. It seems that the more times I answer the same question correctly, the higher the aptitude. This measures whether I really understand the q, or I'm just guessing.
A useful metric would be % correct first time, of all questions asked so far. Most people studying, would like to go through every question, and measure their success by number they got right first time. This percentage is indicative of the score they might get in a real exam. As in an exam there is only one time of answering, this is the most useful measure.