DS20 got the following scores on the proctored practice SAT test. Is their anyway to figure out what this would correlate to for PSAT selection index?
total 1470 (reading + writing 760 math 710)
raw scores
reading 49 out of 52 converts to 37
writing 43 out of 44 coverts to 39
reading and writing 37+39=76 x 10= 760
math no calculator 15 out of 20 calculator 33 out of 38 converts to 710
You add the three components and multiply by two. (37 + 39 + 35.5) * 2 = 223 SI.
@ollie113 but the problem is that the SAT is scored on a 1600 scale and the PSAT is on a 1520 so I don’t think you can use the same formula, can you?
I believe in the instance of a student using the SAT as the qualifying test the NM uses the same formula. I don’t think they scale it, because the College Board indicates the scores should be the same. But I guess to equate to a PSAT you would cut the 39 to 38 for a 221. My D missed 1 on writing and still received a 38 on the 2017 PSAT.
223 SI is the correct estimate for NMSQT. The differences in scales are simply a horizontal shift to the right for SAT to reflect having learned additional content by the time you take the longer test. The two tests are aligned vertically, meaning that your PSAT score is the best estimate for SAT (on the day you took the PSAT). Hope that makes sense!
Actually, that 39 would be a 38 for PSAT purposes because the maximum section score on the PSAT is 38. They won’t give additional points for 39 or 40.
That means the SI would be a 221. (37 + 38 + 35.5) * 2 = 221
My S was absent on PSAT day and had to use the alternate entry method. He took the SAT once and scored 1490, 760V/730 M, broken down as 39 Reading, 37 Writing, 36.5 math.
Per the National Merit folks, his SI for the PSAT is 223. (38 + 37 + 36.5) * 2 = 223
@DiotimaDM so anything 38 or higher on the SAT is a 38 for PSAT purposes, and the other scores stay the same? That’s helpful to know - thank you!
@JBStillFlying Yep, everything stays the same up to 38, and then everything above that is a 38, too. 
Thanks, I was wondering whether they would cap the scores at 38.
If you actually look at the PSAT curves, then you will see that they are usually more lenient than the SAT ones. For example, looking at practice test #1, missing 5 writing questions gets you a 35/40 but missing 5 on PSAT will on average give you a 35/38.
According to the PSAT curves, missing 3 reading, 1 writing, and 9 math (I will say 9 because the PSAT has fewer math questions), you will get a r/w/m of roughly 37/37/34, which is an SI of 216.
When calculating SI from SAT, it is best to just calculated based on number wrong because the curves vary so much. Also take into account that questions are generally easier on the on the PSAT and the PSAT has fewer questions. While the PSAT and SAT are on the same curve, don’t assume a 38. If you assume they are on the exact same scale then it should technically be impossible to get above a 1520 SAT!