You need to check not only the 'C' availability, but the J cabin load. If it's light, it improves the chances that more 'C' gets released. If very full, of course, less chance.
Cheers.
All things being equal (J7 C0), I look for days with D7 I7 or as close to it as possible. Logic tells me that the more seats that are available for sale at discounted prices, the greater the chances are that some of them will convert to C eventually.