I do not know how much time you spent on astrology but you would not be able to set up a chart with the house system we use today unless you new the time of birth within a half hour. Otherwise you would have to use rectification. I still have an interest in the conception chart but no way of really knowing the time for such a chart. Perhaps such a chart would reveal the physical features of an individual since they are developing in the womb. I am not sure how you mean the astral influences. Some explaining would help.
The gist of all above is that we cannot even hope to have the exact time of Conception, unless it happened in a lab.
So I have no idea how ptolemaic astrology can bypass this issue.
By astral influences I mean the time when the newly incarnated soul feels more the influence of the stars, the pattern of the houses as you say, the karmic imprint which is reflected by teh position of the constellations and planets. At Conception, or at Birth?
Or maybe the Conception chart is a reflection of some different karmic inprint than Birth charts? Just speculating.