There is no such problem. Each eye stares at itself in the mirror. (Not necessarily at the same time, if the procedure is done as described.) Those two lines of sight are parallel if the mirror is true, regardless of distance to the mirror.
There may be a different problem: though parallel, those two lines of sight may not be exactly perpendicular to the ruler/mirror. This is not going to make a huge difference.
Suppose the mirror/ruler is rotated five degrees off relative to the plane of your pupils, which is quite a lot. For an pupillary distance of 65 mm, this introduces only a 0.25 mm error on the low side: basically the measurement shrinks by the cosine of 5 degrees which is 0.996.
Pupillary distances are typically given to the millimeter.
> If you are closer then 1 meter to the mirror you should compensate.
If you're 1 meter away from the mirror, you won't be able to read the ruler, at least if you're doing all this because of near-sightedness. :) This technique is best if your face is right up to the mirror. If you press your forehead to the mirror so your head doesn't move, that's probably best. If you're far sighted and can't read the ruler from that close, then you have to back off. If you can't read the ruler at any distance, you may have to do this with contact lenses on.
There may be a different problem: though parallel, those two lines of sight may not be exactly perpendicular to the ruler/mirror. This is not going to make a huge difference.
Suppose the mirror/ruler is rotated five degrees off relative to the plane of your pupils, which is quite a lot. For an pupillary distance of 65 mm, this introduces only a 0.25 mm error on the low side: basically the measurement shrinks by the cosine of 5 degrees which is 0.996.
Pupillary distances are typically given to the millimeter.
> If you are closer then 1 meter to the mirror you should compensate.
If you're 1 meter away from the mirror, you won't be able to read the ruler, at least if you're doing all this because of near-sightedness. :) This technique is best if your face is right up to the mirror. If you press your forehead to the mirror so your head doesn't move, that's probably best. If you're far sighted and can't read the ruler from that close, then you have to back off. If you can't read the ruler at any distance, you may have to do this with contact lenses on.