The best way to answer that is with a tire inflation chart, which I didn't see for the 857. But I suspect you can take a similar tire and apply the same principles.
Here is one, for example:
http://www.maxxis.com/Repository/Files/m8008load.pdf
With the 225/15/D rated, it is about 85% of capacity at 50 psi inflation and 94% at 60 psi. I think that would be a useful guide.