Prescription lenses for shooting glassea

[Sir_SwampThing]

So, for shooting purposes what difference does it make where the datum is, whether at 9 or 12 o'clock ?

I image that the rotation of the lens affects the amount of correction? If the lens is mounted with the datum point in the wrong position, it would possibly increase the astigmatism instead of correcting it?
Or am I way off?

Cheers
Chris