Since the m/z values are very close, there must be a high number of charges on each m/z species.
Let your MW values be Ma and Mb.
Suppose m/z 666.47 has n +ve charges
then Ma = 666.47 x n - nH = 665.47 x n
Suppose m/z 541.7 has n+1 +ve charges
then Mb = 541.7 (n +1) - (n +1) = (n + 1) x 540.7
does Ma = Mb
i.e. does 665.47 x n = 540.7 x n + 540.7 ??
i.e. does 124.77 x n = 540.7 ??
i.e. does n = 4.33
No, but it might = 4.33 x 3 = 13
then Ma = 8651.11, and successive ions are at m/z (8651.11 + 14)/14 = 618.9
m/z (8651.11 + 15)/15 = 577.74
m/z (8651.11 + 16)/16 = 541.7
You should be able to solve from here.