Michael,
While others will follow the water temp, I follow what my bones tell me. Just been doing it for a while and have a feel for when they will be there. Here is an earlier post of mine that will give you some direction.
http://www.tidalfish.com/boards2/viewthread.asp?forum=AMB_AP807600392&id=22086
Disclaimer: No Garuntee Implied. [grin] But pretty damned close.