If doubtful whether to end with "yours faithfully," or "yours truly," or "yours most truly," &c. (there are at least a dozen varieties, before you reach "yours affectionately"), refer to your correspondent's last letter, and make your winding-up at least as friendly as his: in fact, even if a shade more friendly, it will do no harm!
Lewis Carroll [Charles Lutwidge Dodgson] (1832–1898), British author, mathematician, clergyman. "Eight or Nine Wise Words About Letter-Writing" in The Letters of Lewis Carroll, vol. II, ed. Morton N. Cohen, Oxford University Press (1979).