Do we use "which" or "that" when referring to the preceding main clause as a whole?
Which is the correct relative pronoun you should use in your sentence, because you have a main clause ("Peptide adsorption curves may cross"), followed by a non defining, or non restrictive, relative clause that gives additional information or, as per your example, the consequence of what has been mentioned before.
I had never heard of the so called "rule of thumb", but I like it and I'll try to pass it on to others in order to try and sort out this kind of difficulty.