The thing is, just water with a microfiber towel is able to remove almost all the light dirt on plastic and fabric. The ONR helps a little but I would not use it for really heavy dirt. I think that normal dilution is good, but if you want it to be able to remove more, I would probably bump it up to claylube ratio or quick detailer ratio. Do some testing and see what works best for you.
The main role of ONR is to provide lubrication on the paint to prevent scratching while you wash the car. This is not that useful inside of the car. ONR will leave behind some protection in the form of polymers, so that is definatelly a plus. As such, a higher concentration is desirable I would think.
Since the product is so inexpensive, what I do is use 1 ounce in 32 ounces of water for interiors. I can't tell you that is the best possible dillution for this use, but it works well for me.