If you don't have a digital set then you MUST have the appropriate curves for the transducer you are using, preferably in the form of a scale for your set.
DGS is a great system, used properly, If you don't use it properly the results can be worse than useless, even with a digital set, In particular, You do need to understand how to compensate for the transfer correction and the material attenuation (which can be significant, especially in shear wave tests) If you don't have the right numbers for this (and for the probe) you wont get the right results.
The Best guide i've come across aimed at technicians is in John Drury's book , Published by Silver Wing, available from them, or from BINDT, as well as many other sources. Its probably still not completely what you are looking for though.
One point I should add is that if the material to be tested is such that you can use it as a calibration piece (which a piece of plate might well be if it is thick enough that the back surface is out of the near field zone) then the transfer loss can be ignored, and For compression wave testing the material attenuation may also be low enough to ignore.