Most students find their PSAT score, see a number between 320 and 1520, and assume that is what National Merit looks at. It is not.
National Merit uses the Selection Index, and it is calculated differently enough that it should change how you study.
The formula
(2 × Reading and Writing score + Math score) ÷ 10
Your Reading and Writing section scores 160 to 760. So does Math. Run those through the formula and you get a number from 48 to 228.
Worked example. A student scores 700 Reading and Writing, 700 Math, for a 1400 total:
(2 × 700 + 700) ÷ 10 = 2100 ÷ 10 = 210
Now a second student with the same 1400 total, but distributed differently: 650 Reading and Writing, 750 Math.
(2 × 650 + 750) ÷ 10 = 2050 ÷ 10 = 205
Same total score. Five points of Selection Index apart. In a year where a state cutoff sits at 207, one of those students is a semifinalist and the other is not.
What that means for how you study
A point of Reading and Writing is worth twice a point of Math for scholarship purposes. If you are choosing where to spend a limited number of hours and you are anywhere near a cutoff, that arithmetic should make the decision for you.
This runs against most students' instincts, because Math feels more improvable. It often is. But improvement in Math has to be twice as large to move the same index.
About cutoffs
Cutoffs are set per state and move every year, based on how that year's juniors in that state performed. A number you read on a forum for your state two years ago is a rough guide and nothing more.
What is stable is the shape: the most competitive states sit in the low 220s, the least competitive in the low 210s, and the national commended level sits below both. Treat any specific figure as approximate and check the National Merit Scholarship Corporation's own announcements for the year you are sitting.
Which sitting counts
Only the PSAT/NMSQT you take in eleventh grade. Sophomore and younger sittings do not qualify you for anything.
That makes those earlier sittings genuinely useful in a way students underrate: they are the lowest-stakes full-length test you will ever take. Use one to find out how you handle two and a quarter hours of concentration, rather than discovering it on the one that counts.
The window that actually matters
The summer before junior year. It is the only uninterrupted block most students get, and it lands before the October sitting. A student who works through that summer and a student who starts in September are not doing the same thing, whatever the total hours say.