> The time constant RC is based on the voltage rising (or dropping) 67%.

1 - e**-1 = .368, so a one time constant drop reaches 1 - .368 = .632,
making 63% a better value than 67%.

> 67% of 5 volts is 3.35 volts,

It is, but is not relevant here.  The voltage after one time constant = 5V *
e**-1 = 1.84V.

> but if the 2nd invert changes
> state at 4.2 volts, then the pulse width will be longer.

No, it won't.  The exponential decay starts at 5V and goes to 0.  Higher
thresholds will be crossed earlier, making shorter pulses.


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