I suppose you would try to work backward from the known 'properties' of Jesus Christ, and then deduce which planetary configurations would be required for these properties. Alternately, you would use the predicted planetary configurations from an astronomical ephemeris, then devise birth charts for all of them and decide which came the closest to matching Jesus Christ. In either case, you would not do any better than the current debate over the nature of the Star of Bethlehem, which has narrowed the birth dates to one of two possible dates. Astrology is notorious for being subject to human interpretation. No two astrologers are capable of providing the same forecasts for a given planetary configuration, which is what you expect for a system that is not based on an objective reality.