more nuanced, simpler approach to “is box?” determinations

This is a rethink of some of the !3658 (merged) changes, with some deeper consideration of intent and numerical precision. The exact approach in this particular case probably makes little difference. But hopefully this MR reflects a reasonable template for how to get more rigorous about FP usage in general across the code base (or said more bluntly, paying down -Wfloat-equal, -Wfloat-conversion compiler warnings).