You can set up your rubric in some cells like I've done below and use the VLOOKUP formula. Make sure that the scores are on the left, where each listed score is the minimum for the grade it receives. Also note that they're sorted from low (F grade) to high (A grade).
Then the VLOOKUP formula takes your input grade, the range you've set up as above (referenced absolutely - this means it's got dollar signs when we're writing it down, and won't change when you copy & paste the formula), and the "index" of the range we want to get from the formula (the grade is the 2nd from the left, so we put down 2). It looks like this:
If you've got any questions you can find my contact info on my profile.