This is what was done to a raytracer I used. People kept making large-scale scenes with intricate details, think detailed ring placed on table in a room with a huge field in view through the window. For a while one could override the fixed epsilon based on scene scale, but for such high dynamic range scenes a fixed epsilon just didn't cut it.

IIRC it would compute the "dynamic" epsilon value essentially by adding one to the mantissa (treated as an integer) to get the next possible float. Then subtract from that the initial value to get the dynamic epsilon value.

Definitely use library functions if you got 'em though.