From What is the precision of highp floats in GLSL ES 2.0 (for iPhone/iPod touch/iPad)? ...
I get, for instance, on the iPhone 5S, that highp float has a precision of "23". From the documentation (p. 33, section 4.5.2 — https://www.khronos.org/files/opengles_shading_language.pdf ), I'm interpreting that as "the difference between two floating point numbers is at least 2^-23, which is 1.1920929e-7. But I'm fairly confident I'm not understanding that correctly, because I think I've got it calculating differences that are smaller than that by two or three orders of magnitude (and crapping out beyond that).
Am I just getting lucky with my calculations, or am I misunderstanding the value?