How do I generate a formula from best DNN model?

Hello all, I am using DeepLearning to predict salinity values in a local bay. These predicted values are at sample stations throughout the bay. I need to generate a formula from the best model generated to apply to other areas of the bay. I have attempted to use variable importance to do so, but it did not yield accurate results. How would I generate a formula of the best testing model to apply at other locations?