It doens't really matter what you set the gray highlight to because it will just be corrected when you do the linearization. the gray highlight and gray shadow settings just get you closer so the linearization isn't as extreme, but in reality, those settings aren't super critical.
However, if you DID want to edit the gray highlight setting, you would delete the linearization= line because those measurements there are no longer valid since they came from a print WITH the HL setting. You would need to build the curve and do the linearization from that new base quad curve, enter the values and build the new linearized one. Or you could use my software for iterative linearizations...
RB?
Richard Boutwell