I have tried using that site before and wasn't particularly fond of the results. It may technically provide the "proper" image for viewing movies and the like, but it's not necessarily well suited for playing video games in my opinion. I doubt my monitor is the problem here as I haven't faced this issue in any other game.
It probably all comes down to the fact that the in-game gamma slider was not designed to be used with lighting mods, so aside from using Reshade/SweetFX presets on top of STLM, my only real option is to play the game in Borderless Window mode, since that apparently ignores the in-game gamma value altogether; the downside to that, however, is it makes my game stutter to a near-unplayable degree for some reason. Not to mention Essenthy himself has stressed that the mod is "calibrated to be played in Fullscreen, not Borderless/Windowed," which is why I'm asking about the recommended gamma value.