But for the purpose of sensitometry, it's more important to select an exposure that gives you as much data as you need. In the graph I just posted, you can see my normal exposure "ruler" at the top of the sheet.
Christoph gave his film about four stops less exposure than I usually do. So you can see some wasted sample points where the toe is flat. If you gave four stops more exposure the next time you do a test, you will get a lot more information on the right-hand side of the graph.
You can see that I wasn't able to accurately measure Contrast Index (CI) on the 4 and 5.5 minute curves because one of the measurement points I needed (marked with a "C") is off the chart.
So Christoph, whatever you did before... Give four more f/stops of exposure next time.