I'm new to Bioinformatics and I'm trying to learn how to quantify peptides from an iTRAQ experiment. I know with SILAC, everyone uses the log(H/L) ratio because it reflects the relative protein abundance. How do I calculate the relative protein abundance in an iTRAQ experiment (assuming 4-plex)? I'm hoping someone can provide a worked-out example so I can understand what's happening better. I've pulled one row of data from an iTRAQ experiment file. If you need additional explanation of each field in the table, please see this file. It looks like the following:

QueryPrecursorMz             400.5555
OriginalPrecursorMz          400.5555
PrecursorError(ppm)          0.45712903
QueryCharge                  3
OriginalCharge               3
PrecursorScanNum             1
PrecursorArea                9.55E+08
PrecursorRelAb               0.00012481
RTAtPrecursorHalfElution     45
PeptideSequence              +144.102HNQRPTFR
AmbiguousMatch               0
Protein                      gi|160420317|ref|NP_001104026.1|(pre=K,post=Q);gi|116063573|ref|NP_001447.2|(pre=K,post=Q)
DeNovoScore                  68
MSGFScore                    68
Evalue                       8.58E-05
Qvalue                       8.48E-05
PepQvalue                    1.49E-04
PrecursorPurity              63.4,72.8
FractionDecomposition        94.7,94.7
HCDEnergy                    19.0664520263672eV
iTRAQ114                     16563.3/0.09
iTRAQ115                     118556/0.10
iTRAQ116                     157171/0.09
iTRAQ117                     119378/0.09
iTRAQFlags                   I
iTRAQTotalAb                 411668
iTRAQFractionOfTotalAb       0.136185