“An individual’s facial midline is defined by the soft tissue symmetry – base of the nose, nasal apex, center of the philtrum and central point of the chin -, and the upper dental midline is evaluated by locating the tip of the gingival papilla between the maxillary central incisors. The gingival papilla should be located below the center of the philtrum of the upper lip.” (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5278933/)
Where you sit to determine if a midline shift is present really depends on what you are comfortable with.
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