Measuring for mirror self potrait

You do need to close one eye to sight it properly. I focus on the mirror so you can actually see the size of the image you're measuring.

You could draw on the mirror itself and if you draw with a greasy or powdery substance, you could transfer it onto paper, thus revering the image of the mirror image. This would make it the right way round.

You could use scale dividers. https://www.artsupplies.co.uk/item-derwent-scale-dividers.htm The image of your face seen through the mirror will be smaller than the image you want to paint on canvas. To start with, set the adjustable hinge so that the ratio of the small arm to the long arm is about 1 to 2. Look in the mirror, and measure the distance from the top of the head to underneath the chin by placing the points of short arm on the surface of the mirror. Now place the points of the long arm on the surface of the canvas. Does the top and bottom of the head fit neatly within the canvas ?. If it doesn't, adjust the position of the hinge, remeasure on the surface of the mirror, then check again with the long arm on the canvas. Once you've got the ratio right, mark the top and bottom of the head on the canvas with pencil or paint. Make a few more measurements - top of head to eyeline, eyeline to bottom of nose, bottom of nose to lips, lips to under chin, width of head just above ears. Mark them on the canvas. There's not much point in making any more measurements at this stage. As the painting progresses, if the proportions don't look right, you can always make some more check measurements. I'd set up the mirror so that you have a full view of the front of your facet. Sit next to a window so you have a strong sideways light casting shadows over one side of the face. Position the canvas so that it's close up to the edge of the mirror. That way you don't have to turn your head much It's a challenge to do a self portrait through a mirror if you want to have a three quarter view of the face, as you're continually turning your head as you move between the mirror and the canvas. For skin colour and hair I use a simple palette of Cadmium red, raw sienna or yellow ochre, and black and white. It gives a huge range of convincing skin tones

Or get a selfie-stick?