I have a 100 x 75mm fence post and want to cut a notch for a 70 x 45mm rail. If I cut the notch the full thickness of the rail it would only leave 30mm on the post. I'm assuming that's too much. Could I cut half of the 45mm thickness of the rail from the post and half from the rail? That would leave over 50mm on the post.
Based on the understanding that the second reference to post means a reference to rail, your approach is sound. You'll be removing the minimum amount from both components while retaining the maximum strength for both.
Removing half from each means that the rail will be offset in the post and not flush with either edge, which is not particularly a problem, unless the esthetics are critical.