In the first edition of Dan Margulis' Photoshop Lab Color, he touches on this subject on page 99 with this comment related to the midpoint Lab L value and RGB(128) by noting that "...the higher the gamma, the darker the midpoint. (My emphasis) Plus, we need to know that almost everyone uses one of two gamma settings: 1.8, which is the tradaditional Macintosh setting, or 2.2, the traditional PC setting. Both sRGB and Adobe RGB both employ 2.2...."
There's more, where the L value is set from 54 to 61 (the variable is never at 50). Dan delves deeper into the subject in Chapter 13, as well.
So, for starters, I would strongly recommend that first edition. Its ISBN No. is 0-321-35678-0
BTW: If you are interested in going deep into Lab Color, his later edition is just as valuable.
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