Not sure if I quite understand what effect you're trying to achieve but you could try:
1) de-saturating the image (turning it to black and white) If you've not already done so
2) using the "posterIze colour" effect and choose 1 colour. This will force just one colour (black) against white.
Adjustment using "levels" of the b&w image (after step 1) will influence which shades will be made black in step 2.
Thanks! I’ll give that a try. Basically, I’m trying to create a 1bit image for an LCD screen font. …so each pixel is either on or off, (black or white).
In addition to what Blackbird said, try also FX (3rd icon from the top right) > Stylize > Threshold. The default settings should serve you well. (It always reminds me of an old monochrome computer monitor screen, but in high-resolution and in black and white.) You may have to play with the Threshold setting depending upon how detailed the image is.