ORP and Ozone Question

[The Codfather]

Looking for a bit of advise on some reading that I am getting. Here is just a bit of background:
!50 gal display and 60 gal sump
3 x 250 watt metal halide (7 hr cycle)
SPS dominated
10 assorted fish, damsels, gobies ,couple of clowns, etc
BK 250 SM skimmer
I started running ozone at about 10-15 mg/l, controlled and monitored of course, and initial reading with orp were anywhere between 280 and 320, pending of course what time of day it was.
In the following weeks, orp started to rise, slowly, but usually a couple of points a day. About a week ago, it broke 400. I shut off my ozone a few days ago, and the reading do fluctuate but are still hovering around the 400 mark. I guess my questions are:
1 For those who are using ozone, what is your high limit?
2 How much ozone are you using on your system?
3 Have you noticed any problems arising for the use of ozone?
thanks,
Bob