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My first idea was also blendif but I think you could just try using magic wand tool with deselected "contiguous" with tolerance set to 5 and clicking a pixel with L50 should select all pixels from L45 to L55 (best to try in L channel directly)

EDIT: Nope  It's not working as I suspected

EDIT2:

This is longer solution but as far as I tested works 100%

1. Create Threshold adjustment layer and set its value to - your number * 2,55 - so 45*2,55 = 115 (the closest full number)

2. using shortcut Cmd+Shift+2 load luminosity as selection

3. Save selection as channel using menu: Select>Save Selection > Ne Channel

4. Change value of Threshold adjustment layer to you second value 55 * 2,55 = 140

5. load luminosity as selection

6. Remove selected pixels from channel you created earlier

This way you have a mask which should contain all pixels from L45 to L55 which you can load as selection just by ctrl+click on channel miniature