The maximum value is only really important if you’re doing highlight reconstruction, otherwise you can just do exposure compensation. You can probably find the raw black/white points by using exiftool on one of the input raw files if this came from a mirrorless or SLR.
That said, looking at the discussion over on the iM site, something is weird with the FITS output causing it to have negative values which is extremely unusual, especially since they’re almost universally negative. (Negative values in RGB can sometimes indicate an out-of-gamut color, but something else is going on here.)
BTW, the tiff looking dark is not actually correct, per some of the discussion over at Color management in Siril, need help and refactoring. and Siril needs distortion correction in stacking - #31 by flint - it really should have an ICC attached that indicates linear data.