Delta T should be 5 to 6 deg's. For -20 DegC application the ideal SST is -26 Deg C. ( That is the return suction line temperature at suction header.).

Check the super heat. genearlly we should maintain 8-10 Deg. at Evapoartor suction. For Air conditioning 8 - 12 Deg is ideal.
Tx and regards
Manoj Menon.