Hey, @nahgul! I agree with @Kristian Nee, and I gotta say staying in the same pose for a long period of time can be very challenging even for professional models, let alone attempting to keep going back to exact same pose several times when you’re your own model in the mirror and your main goal is not to pose, but to draw. It can definitely get exhausting pretty quickly, which then only makes it harder to find the exact same pose again and again. Plus, attempting to measure “perfectly” will lead us to a good copy of what we see in the best case scenario - which can be helpful as that “weight-lifting training” when we’re specifically seeking to get better at measuring techniques, but, if it’s not combined with other aspects of art-making, won’t do much to help us grow as artists in the long run. One thing @Court Jones’s caricature course has taught me is that a good likeness is not about copy, but interpretation: highlighting unique features of one unique person through a unique artistic vision. In realistic portraiture, we won’t take this as extremely as in caricature, of course, but what I mean is that you don’t need to copy everything exact in order to get good likeness. With all that said, my recommendation when working from life would be to let go a bit of that “measuring precision”. You can and should use measuring when you feel that it helps, but don’t be attached to just that. Use what you see in the reference, but also what you know (based on previous studies of the head and face anatomy) and what you feel (based on your own artistic sense and interpretation of your subject, whatever it is). And if you believe you need to work on improving measurement techniques, you can do it in a separate moment with this specific goal to get the best out of your training (and, preferably, with a reference that won’t move, since it should make it easier to focus on measuring). :) Hope this helps!

One obvious one would be to put a photo next to the mirror. That way you can still keep the "life" aspect from the mirror, but still maintain the information of having a photo. The next thing I'd say is to make sure you're well lit. It's hard to invent something from your head if you don't have a road map on what to do. The more you can make the drawing linear, the easier it'll be to achieve definite shadow shapes and from there it's just a matter of refining those until they hit that likeness you're after.