My understanding with vitamin C is that if your body is stressed it usually needs more. And if your body needs more, it won't throw your body off kilter to up the doses, it's simply being used up by your adrenals or immune system or whatever needs the C to help you feel better. So there shouldn't be much left over to throw off your pH.
As a rule of thumb, once you get loose bowel movements you should back off a bit on the C. But until then, it's okay for your body to get big doses because it's using it up!