If this were shade 7 I would just give you a P2 curve and be done with it, but because this is 6, it's a required shade.
You have two ways to go about doing it. One way is hard and one is easy.
You will need a bottle of shade 6. You will need to dump the current ink you have, then put some distilled water into the cartridge, then dump that out (as much as possible).
Then put shade 6 in, and print a purge sheet (use the LK Purge file from your QTR Purge folder) on some scrap paper (24"x90" in size). I generally use a roll of Canon Bond paper.
Print at 1440dpi bi-directional.
If you don't have an ink bottle, order a shade 6 bottle online and it will go out in the morning tomorrow. We are past the cut-off for overnight same-day today. If you call us tomorrow before 10am, Wells is in and he can get you out a bottle rush.
If you have a bottle of shade 7 and a bottle of shade 5, mix them 1-1 to make a "not perfect but passable" shade 6.
Put this new DIY shade six into your cartridge and flush like I described in the "easy way".
Then re-linearize your ExFiber .quad.