When you first start preparing for a standardized exam, you usually take an entrance test. It’s usually lower than what you had hoped for, and then you wonder, “How many points can I expect to gain?” That is a very good question, and also a complicated one. It doesn’t make sense to measure improvement in “points” or even in “percentiles”, because those units are not equal for all students (it’s much easier to gain 5 points or 10 percentile points at the lower end of the scale than the upper end). The metric I use instead is the “Improvement Score”, IS, which is based on the percentage of students ahead of you that you can pull ahead of. You can use your entrance diagnostic score and the average improvement score (IS = 50) to get a very good ballpark estimate of your exit score after an average program of study.
Use this spreadsheet: How much will my score improve on the SAT / ACT / GRE / LSAT / GMAT?
View this video for an explanation: