There was actually some debate about whether stylus should lighten colors relatively to their luminosity or just "add white" on top of it.
Anyway, you want to replace the luminance value in the hsl scale, so you can decompose your color and only keep the hue and saturation, and force the luminance value you want:
c = #123;
n = hsl(hue(c), saturation(c), 86);
Here 86 is a percentage.