I too use the $150 - 200 a night rule.
Rememeber that when you use points for a cheap room, you not only loose the points you spend but points you could earn.
So If I can get a $100 room for two nights.
I can either spend 35K points (cat 4) which is about $ .006 a point. Or spend $200 and save 39600 points (35K + 4600).