Did you just push the end hook against one drop?
Use the tape like a ruler with the "start point" at the 10mm. (or other convenient place like 100)
Lay it IN the drops and measure to the other side, carefully lining up the 2 marks at each drop.
Subtract the 10mm. (or??)
Some tapes have the hook bent badly and give very erroneous readings when taking an "inside" measurement.