How to use lookup or match function to find the closest value in an array according to a calculated number

Hi All, I have a variable x that can be calculated by a function. I also have an array to determine the practical numbers that can be used in this variable. I want to use lookup information to find the closest information from the array based on the calculation results. However, it seems like the guides in PTC Mathcad Help cannot really solve my problem. Would anyone take a look at the attached worksheet? Thank you!