It's a long and complicated topic, and not one I'm going to tackle during the peak period, but ORP is a fairly useless water value, with respect to chlorination control on outdoor stabilized pools.

More for my benefit, than yours, I collected below some of the key threads on PoolForum that relate to the topic of ORP/pH control. You're welcome to dig in, if you like.

http://www.poolforum.com/pf2/showthread.php/10017
http://www.poolforum.com/pf2/showthread.php/4236
http://www.poolforum.com/pf2/showthread.php/15706
http://www.poolforum.com/pf2/showthread.php/10303
http://www.poolforum.com/pf2/showthread.php/5105
http://www.poolforum.com/pf2/showthread.php/11936
http://www.poolforum.com/pf2/showthread.php/1064