Quomodo signum crucis sit exprimendum
The Catholic Encyclopedia's article on the sign of the cross
implies that, for Romans at least (both east and west), right-to-left was the original form (along with two fingers (so the Old Believers were right all along!)).
At this period [i.e. the early Middle Ages] the manner of making it in the West seems to have been identical with that followed at present in the East, i.e. only three fingers were used, and the hand traveled from the right shoulder to the left. The point, it must be confessed, is not entirely clear and Thalhofer (Liturgik, I, 633) inclines to the opinion that in the passages of Belethus (xxxix), Sicardus (III, iv), Innocent III (De myst. Alt., II, xlvi), and Durandus (V, ii, 13), which are usually appealed to in proof of this, these authors have in mind the small cross made upon the forehead or external objects, in which the hand moves naturally from right to left, and not the big cross made from shoulder to shoulder. Still, a rubric in a manuscript copy of the York Missal clearly requires the priest when signing himself with the paten to touch the left shoulder after the right. Moreover it is at least clear from many pictures and sculptures that in the twelfth and thirteenth centuries the Greek practice of extending only three fingers was adhered to by many Latin Christians.
Est autem signum crucis tribus digitis exprimendum, quia sub invocatione Trinitatis imprimitur, de qua dicit propheta: Quis appendit tribus digitis molem terrae? ita quod a superiori descendat in inferius, et a dextra transeat ad sinistram, quia Christus de coelo descendit in terram, et a Judaeis transivit ad gentes. Quidam tamen signum crucis a sinistra producunt in dextram; quia de miseria transire debemus ad gloriam, sicut et Christus transivit de morte ad vitam, et de inferno ad paradisum, praesertim ut seipsos et alios uno eodemque pariter modo consignent. Constat autem quod cum super alios signum crucis imprimimus, ipsos a sinistris consignamus in dextram. Verum si diligenter attendas, etiam super alios signum crucis a dextra producimus in sinistram, quia non consignamus eos quasi vertentes dorsum, sed quasi faciem praesentantes.
The sign of the Cross should be expressed with three fingers, because it is impressed under the invocation of the Trinity, of Whom the Prophet says: “Who hath poised with three fingers the bulk of the earth?” (Is. 40, 10), so that one descends from above underneath, and from the right passes to the left, because Christ descended from heaven to earth, and he passed from the Jews to the Gentiles. Nevertheless some make the sign of the cross from the left to the right, because we must pass from misery to glory, just as Christ passed from death unto life, and from hell to paradise, especially so that they may sign themselves and others simultaneously in one and the same way. It is true that, when we make the sign of the cross over others, we sign ourselves from the left to the right. But if you pay attention carefully, we also make the sign of the cross from right to left on others, because we do not sign them as if they were turning their backs (on us), but as they face us.
Pope Innocent III (1160 –1216), De sacro altari mysterio, II, 45 (PL 217, 825D)