Length-matching microwave differential traces
So, if I wanted to keep the delayed amplitude within 1% of the value it should be (for arbitrary phase), I should keep the length difference less than 1% of the electrical length.


I'm not clear how you got to this rule, but I'd expect to have a rule that gives the length difference as a fraction of the signal wavelength (in the transmission line) rather than as a fraction of the trace length. 

The reason is if you consider one of the two traces as "ideal", then you want to know how out of phase the signal on the other trace is at the receiving end of the trace, not at the source end.

That said, your trace length isn't that much different from one wavelength, so using a 1% rule will probably work well.


  Can I get away with more? 


Probably.

Depending on your requirements, the EMI (radiation) produced by common mode signals created by length mismatch might be a more important determiner of the maximum mismatch than your performance requirements.