John
The acceptance chart has several factors to take into account to get to the ratings..Class A, Class B, Class C, Class D.
If you have gotten that far then it is a matter of evaluating the length of your discontinuity by using the 6 dB drop method to determine if it is acceptable per those Classes.
Now if you need to figure which Class something you find with UT is in, then peak up on your indication and then take out dBs until you get the signal to the reference line, now note how many dBs your gain is at.
This is your Indication level(a), take that and subtract your reference level(b) from that figure, then subtract that answer from your attenuation figure(c)[attenuation = Sound Path minus 1, times 2] and that will give you your Indication rating(d). Now look at your table 6.2 (for statically loaded structures) and find your material thickness and transducer angle to see which class your Indication rating(d) falls into.
ie. 70* transducer with 1/2" material
Class A = +5 or lower
Class B = +6
Class C = +7
Class D = +8 and up
If I mis understood your question post the question again.
John Wright