Unless you're an engineer running your own calculations, only fill to the manufacturers recommended fill pressure...
No one can give a broad answer that covers all brands/models of air guns, what may work for one may not for another...so its better to be more specific and state which brand/model you intend to attempt this one before even obtaining a genuine answer.
There are modifications one can do to increase their fill pressure, but its in your best interest to be educated on the matter, theres more than 1 calculation to conduct, as the regulator only reduces the pressure in a portion of the rifle, but you still expose the fill cap and components as well as the tube itself to higher pressures...
-Matt