Read Clause 6.26...(the whole thing)
Several key things.....
6.26.6.3 adjust the gain so the signal of the indication is at your reference line(50%FSH).
6.26.6.4 figure your attenuation
6.26.6.5 follow the algebraic expression that fits your machine(mostlikey the one for gain)
6.26.7 determine the length using the 6 dB drop method(see 6.31.2 for angle beam)
Now go look at Table 6.2 and see where you indication falls. Don't for get to use the categories listed at the bottom left of the page under that Table6.2(and use the notes)