Occasionally, you will see a warning dialog that informs you that a "Reaction clipped."  That means that one or more of the color components moved outside of the testable range.  This makes it impossible to calculate the reaction from these two ingredients.

You can solve this by doing a three-way test.  To do this, select a third ingredient that has known reaction values with the two ingredients you are attempting to pair, and which will adjust the color component that was clipping, so that it is no longer doing so.  Combine that with your two ingredients.  In Desert Paint Lab, you'll want to select a known pairing in the first two boxes, and the other ingredient in the third.  Desert Paint Lab can then do all the math to figure out the mystery reaction.

In many cases, it may be easiest to go back and do these three-way tests after you have finished all of your other testing.