In USP <857> (update Dec. 2022), two possible procedures are described for checking for stray light:

Procedure A
Here the sodium nitrite filter (UV11) with a path length of 10 mm is measured against the sodium nitrite reference filter (UV11H) with a path length of 5 mm. The stray light value can now be calculated from the absorbance maximum obtained, using the following formula:
S_λ = 0.25 x 10^-2∆A
The following acceptance criteria apply:
∆A ≥ 0.7 Abs and S_λ ≤ 0.01

Procedure B
Here the sodium nitrite filter (UV11) with a path length of 10 mm is measured against the reference filter (UV12) filled with pure water. The measured absorbance value at the recommended wavelength 340 nm should be ≥ 2.0 Abs.

Note
Procedure B of USP 857 corresponds to the procedure of the European Pharmacopoeia