SMF-28 attenuation vs wavelength

And your problem with that is what, precisely?

SMF-28 is a bit old-school. Not sure where to dig up a product data sheet for it at this date. Ah, here's a blast from 2002 just for that.

for the worst case, take the non-premium fiber number at 1550 and apply the attenuation .vs. wavelength adjustment to get 0.27 dB/km (and look at the typical fiber graph to extrapolate downward towards 1600)

SMF-28e+ has a currently available datasheet, which pretty much says you probably have an acceptable number already. The water-peak reduction is at shorter wavelengths than you are asking about, so it probably has little effect .vs. old SMF-28, if that is what you have.

Those are maximums, for design purposes - a link designed with those numbers and correctly installed should function. Actual attenuation could be less without violating specs.

If you have a need for "actual" rather than "maximum" attenuation, you'll have to measure it, and reduce all points of measurement error as much as possible if you feel a need for greater precision. That will tell you what the piece of cable you are measuring will do - it may not generalize to other cables well.

If you have a need for greater reliability, assume 0.25 dB/km and build in some margin.

If you have some other need, please explain it more completely, and you might get a better answer.

This link discusses attenuation .vs. wavelength somewhat generally, with graphs you might find helpful. Paste it together if it gets mangled in posting.