Mechanic reports about the confusion caused by the (SRM) Structural Repair Manual data that led to his not properly evaluating a dent on the right horizontal stabilizer leading edge of a B737-400.

Narrative

Confusion caused by Structural Repair Manual data difficult to read and evaluate. I evaluated a dent on the right-hand horizontal stabilizer leading edge on Aircraft X. As stated in my log page signoff; I gave the dimensions of the dent as 3 inches long; 1.5 inches wide; and .125 inch deep. Due to the confusing data of the Structural Repair Manual; I did not see the unmarked note on Page 107 of allowable damage 1; that gives an A/Y measurement and directs you to detail viii. I first went to Page 104 that directs you to paragraph 'p' for the dent. I then read the list under paragraph 'p' and came to what I then thought was the correct A/Y ratio to use. At the time I did not notice the distinctions between the zones listed. Having used what I thought was the correct a/y ratio; I was in fact using incomplete data. It was not until I was notified later about this discrepancy that I realized my error. At the time I did not realize the dent was in Zone 3 and; therefore; needed further action taken. At no time did I notice that the dent possibly overlapped a rib section; until so informed. As I have already stated; the Structural Repair Manual is extremely difficult to read and understand; so I will now seek further guidance from other sources in the future.