Precursor Mass Correction:

You are given the following (ideal nominal) MS/MS spectrum for a doubly charged peptide: S =
 {227 (b2), 276 (y2), 330 (b3), 375 (y3), 490 (y4), 544 (b5)}.

 Find the precursor mass with the algorithm in LU8A.

Solution: 276+554-2 = 818 gives (among others) the most (4) matches in the reverse spectrum. Therefore this is the most probable mass.
Please note, that the fragment ions theirselves are singly charged. Therefore you do not need a division by charge after adding complementary m/z values and substracting 2 (protons).