Focus At What Distance When Shooting Reflection In Mirror?

What is the correct distance at which to manually preset focus, to photograph a reflection of something (somebody) in a mirror?

What is the effect, if any, of the plane of the camera's sensor not being parallel with the mirror? i.e. shot at an angle to the mirror, not straight on.

I cannot see the subject, because I will be the subject (self portrait), and I won't be behind the camera when photographed. I will have the camera tripod mounted and will fire it remotely.

I could figure it out by trial and error, but I'd like to know any applicable theory and the theoretically correct distance anyway.

How far does the light travel to get to the camera?

Add up camera to mirror and mirror to subject distances, you are effectively shooting something an equal distance 'behind' the mirror as the subject is in front of it.

Easiest way would be to place something where you intend to be and focus on it, then simply take it's place and fire the shot.

OK. Cool. Thanks. I'll probably measure it out with a tape measure.

The easiest way would be to stand something on the spot you will be standing, and use that to set the focus on your camera. But Sandpiper is right if you want to measure it. It's the combined distance of camera to mirror and mirror to subject.

It doesn't matter if the sensor is parallel to the mirror. The only thing that matters is if the sensor is parallel to the subject's reflection. In other words, let's say you put your camera to the right of the mirror, with the sensor at a 30-degree angle. To be seen, you'll need to stand at the left. If the left (farther from the camera) side of your body is closer to the mirror, and you yourself are at a 30-degree angle, the sensor will be parallel to your reflection and everything will be in focus.