The `with` style is fine with me.  I'm not too dogmatic about always using verbs for function names.  My rule is to think about how the function will be used and see if it reads well.  `sed = sed.withFlux(bandpass, flux)` reads reasonably well to me, so if you like that, go with it.  

Possibly slightly better to swap the order of `bandpass` and `flux` though in that case, since then it would kind of read `sed withFlux flux [in] bandpass`.  But that's niggling at best, so choose whatever version you like most.