  We consider the uncertainty of theoretical calculations for a specific
difference of the hyperfine intervals in the 1s and 2s states in a light
hydrogen-like atom. For a number of crucial radiative corrections the result
for hydrogen atom and helium ion appears as an extrapolation of the numerical
data from medium to low Z. An approach to a plausible estimation of the
uncertainty is suggested using the example of the difference $D_{21}=
8E_{hfs}(2s)-E_{hfs}(1s)$.
