Thanks. Most CRT shader presets just aren't well suited for anything newer than CGA monitors since they are mostly made for replicating the look of an early game console on a (cheap) TV. They do great in that regard, but that's just not how VGA/SVGA looked on a PC monitor. For making DosBox look like something from the VGA/SVGA era shaders have to be modified. Most importantly no VGA capable monitor ever had blatantly visible scanlines for each pixel of a 200/240p resolution, even the oldest VGA monitors drew two scanlines per pixel at 200/240p (double scan). When I think back to monitors I had in the 90s/early 2000s, I don't really recall them to have any noticable scanlines at all.
Anyway, the look of the corners can be changed with the CURVATURE and CORNER parameters just above the section with the added parameters for the luminance gain. I didn't change any of that, so these controls work just as they do with the unmodified lottes-fast shader. BTW: The lottes-fast shader also allows you to simulate a Trinitron aperture grille instead of a RGB dot matrix mask, IIRC you would have to set MASK to 1 instead of 3 for that. Caveat: doing so will break the carefully adjusted parameters for my added luminance gain since with the Trinitron grille the shader looks brighter per se.
The strength of the luminance gain can be adjusted with the FINAL_GAIN_MULT parameter or by lowering FINAL_GAIN_POW. Lowering the latter would leave the maximum strength the same, but darker parts of the image would be affected more. That's probably what you want to do.
Be careful though, the general idea behind the gain besides providing an output image that looks more similar to the input was that at full white the aperture mask becomes almost (but not entirely) invisible, which looks as if very bright colors outshine the mask. Even small changes to the parameters can break that impression by either making the max values too bright or not bright enough.