Calculating Delta MFCC, correct formula?
The way you compute MFCCs and derivatives has little-to-no influence on the final result, so just pick one and go ahead. Although when applied to speech recognition and similar problems the same algorithm should be used for training and decoding. Thus if you use a pre-trained model you have to know how features for its training were computed. If not sure, you can always take a look at real-world examples - CMU Sphinx and Kaldi.