Over the last six years I have had very good success predicting the outcome on the Regents exam prior to the student taking the exam.
I provide the spreadsheet to do this when you sign up. All you need to do is fill in the results on three exams and the Lab Practical. If you do that it will predict the raw score. You can then use the conversion chart that the Regents provides to determine what that score would be. There is a very high correlation between these predictions and the actual scores (approximately 0.8 on a scale from 0 to 1 where 0.9 is nearly perfect correlation).
This has several benefits: first you can see how you will do and decide whether or not you are ready and second, you can also see how improving one of the three major exam scores will impact your overall score. That way you can easily see what you need to study.
There is a second way to predict the outcome, the correlation is lower (around 0.65) but it may be more useful on a day to day basis. You total up the total number of points the student has gotten on all assignments (simple if you keep track on a spreadsheet). 10,000 points on these assignments is the point at which virtually all students pass the Regents exam. 15,000 points is the point at which many students will score in the 90s. There are other factors that will affect the outcome (does the student read above grade level or below grade level, and how has the student done on previous exams).