The Selection Index is twice your PSAT Reading and Writing score divided by ten, plus your Math score divided by ten. With both sections at the 760 maximum that gives an index of 228, the ceiling. Because Reading and Writing is counted twice and Math once, a ten-point gain in Reading and Writing moves your index by two while the same gain in Math moves it by one.
The double weighting on Reading and Writing is the part most students miss, and it changes where preparation time should go. If your index sits a few points under your state cutoff, the cheapest route up is almost always the Reading and Writing section, because every ten points there are worth twice what the same gain in Math is worth.
The full picture, with the surrounding detail, is in PSAT National Merit Cutoffs by State + What It Predicts.