Use the value of the attenuation coefficient written in the NeXus files in sans_sheet
The attenuators in D11 were changed in 2018. This was taken into account by adding a new table of attenuation values to sans_inst_init.pro and handling the index of each possible attenuator in read_hdf.pro (attenuators 1-3 before 2018 and attenuators 4-10 in 2018 and afterwards). However, the values in 2019 have changed again so either this has to be extended again (using attenuators 11-17 for 2019) or sans_supl[26] modified to use directly the attenuation coefficient whenever this value is available (this is written in the NeXus files for D11 since cycle 181 and read by read_hdf.pro in parameter 29).
Note that on D22 there are attenuator and attenuator2 and the equivalent field is called attenuation_value.
The same for D33, with the additional complication that when the attenuator is out, the attenuation_value is 0 (instead of 1, as for D22 or D11)