Real life has the E3 series: 1, 2.2, 4.7, and then 10, 22, 47, 100, 220, 470, 1000, etc. Etc.

EEs would recognize these values to be the Preferred Resistors values for projects (though more commonly the E6 series is used in projects, the E3 and E1 values are preferred)

That's 3 values per decade, which is slightly more dispersed than a FP4 that consists of 1 sign + 3 exponent + 0 (implicit mantissa 1 bit).

Or the values -128, -64, -32, -16, -8, -4, -2, -1, 1, 2, ... 128.

Maybe we can take -128 and call that zero instead, cause zero is useful.

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Given how even E3 is still useful in real world electrical engineering problems, I'm more inclined to allocate more bits to the exponent than the mantissa.