For calculation the gradient (temperature rise) of transformer winding you need a lot of technical data about your transformer. You can follow calculation procedure below which submitted in a reference book (Spotlight on Modern Transformer Design written by Pavlos S. Georgilakis) to compute the average gradient between the oil and the LV winding, the average gradient between the oil and the HV winding also the average oil temperature rise of a 630 kVA distribution transformer having the input data shown in Tables. 
