This is easily remedied. Mark the vial and the neck of the Checker. Align these marks every time the vial is inserted to ensure the same area of the vial is in the light path.
For about the price of one Hanna test vial and cap, you can buy a dozen three dram vials with caps. You will likely find a vial without these flaws where the light passes through. There is also a good chance that you will find two vials that are similar. You can then use one for the "C1" calibration and one for the test sample for the "C2" measurement. With this approach, you can have the sample mixed and ready to go and then do the "C1" calibration, thus avoiding the rush to prepare the sample after the "C1" calibration.
