You don't really need to change the recipe much. The pH targets are more to point out the change in pH from step to step, not some commandments to be followed blindly. If your milk comes in at 6.6 instead of 6.65 or 6.7, then focus on the change in pH and how it works together with your culture. All recipes must be customized to fit your milk, environment, and cultures. You typically need to tweak the time or acidity schedule, or something else. But if you use the approach of change in pH from step to step, it should work.
Please note, Stilton makes a LOT of curd. Be prepared for the volume