In general the before-after variation of DA, DB are modest. In this case the values of DA, DB to be used in the calculations can be taken as their arithmetic average before-after the sounding. However:
If there is an appreciable variation of DA, DB before-after the test.
In this case a “logical” average is better than the “arithmetic” average. For instance, in case of a site composed of a soft layer overlying an hard layer, choose DA, DB closer to the initial DA, DB, that presumably were in effect while investigating the soft layer, mostly in need of adequate DA, DB.
If there are many negative Ed values.
Then the used values of DA, DB are too high. In this case reduce DA, DB so that there are no negative Ed values (or may be only one or two negative Ed values).
More information in the Note: “On the calibration of the DMT membrane“.