I'm inexperienced with ORP, so take my comments with a 'grain of salt', and this is simply based off what I have researched....
I have heard that above 350 and below 450 is the 'zone' for ORP numbers. However, even this is not 'set-in-stone'.
Additionally, I have seen most people utilize the ORP number(s) as a guide to trend tank information, vice a 'set' number to achieve. Basically, ORP 'trending' may provide you with information as to something going on in your system.