PSAT score and NMSF Selection Index are computed using different formulas. Therefore, there is not a one-to-one correspondence between the 2 numbers. The PSAT score calculation gives twice as much weighting to the M subscore:
PSAT = 10 * ((2*M) + V + CR)
NM S.I. = 2 * (M + V + CR)
I have crunched the different combinations of subscores and determined that these are the possible ranges of the numbers:
PSAT | NMSF Selection Index range
1200 | 176 - 184
1210 | 178 - 186
1220 | 178 - 188
1230 | 180 - 190
1240 | 180 - 192
1250 | 182 - 194
1260 | 182 - 196
1270 | 184 - 198
1280 | 184 - 200
1290 | 186 - 202
1300 | 186 - 204
1310 | 188 - 206
1320 | 188 - 208
1330 | 190 - 208
1340 | 192 - 210
1350 | 194 - 210
1360 | 196 - 212
1370 | 198 - 212
1380 | 200 - 214
1390 | 202 - 214
1400 | 204 - 216
1410 | 206 - 216
1420 | 208 - 218
1430 | 210 - 218
1440 | 212 - 220
1450 | 214 - 220
1460 | 216 - 222
1470 | 218 - 222
1480 | 220 - 224
1490 | 222 - 224
1500 | 224 - 226
1510 | 226
1520 | 228
S.I. | PSAT score range
190 | 1230 - 1330
192 | 1240 - 1340
194 | 1250 - 1350
196 | 1260 - 1360
198 | 1270 - 1370
200 | 1280 - 1380
202 | 1290 - 1390
204 | 1300 - 1400
206 | 1310 - 1410
208 | 1320 - 1420
210 | 1340 - 1430
212 | 1360 - 1440
214 | 1380 - 1450
216 | 1400 - 1460
218 | 1420 - 1470
220 | 1440 - 1480
222 | 1460 - 1490
224 | 1480 - 1500
226 | 1500 - 1510
228 | 1520