If you are preparing for PSAT/NMSQT test, you most likely will simulate test day by printing and taking official PSAT/NMSQT practice tests from here. However, this page suggests you score by hand, and provides 9-page pdf with instructions how to score your total result, subscores and cross-test scores.
While calculations are not that difficult, they require you to be very careful in the process, and yet it is easy to make a human error.
Calculators below transfer scoring guide logic into the code. You only have to mark INCORRECT ANSWERS in checkboxes and code will calculate all your scores for you (I optimistically assume that your number of correct answers is greater than number of incorrect answers, so in my opinion, it is more usable to mark incorrect ones).
Note that for official practice test #2 below, the maximum score you can get for Analysis in History/Social Studies cross-test is 37, not 38, no matter what. This is because scoring guide has an error - for this cross test it lists only 31 questions, not 32, thus, maximum score of 38 which you can get for 32 correct answers could not be reached.