For avascular necrosis of the hip what your doctors suggests is right. Using crutches or a tripod stick will be better option. This will help in less weight bearing on left side. For shortening, you can measure from umbilicus to medial malleolus of both heels, keeping both legs straight in lying position. Whatever is the difference, minus 1/2 inch will be correct compensation. If there is pain in other joints, clinical examination from a physiotherapist is advisable. Wish you all the best.