I'm trialing a new technique. Book J well in advance, this gives me security that'll have a comfortable seat and flexibility. Night before flying, when I know definitely I'll be travelling, check the flight. If it's obviously lightly booked, book Choice and select a front row seat with no-one in the middle. Cancel J flight. If flight's looking full, stick with Plan A.
Obviously not good for accumulating SC's but works financially. Now it's purely $ based, there's no incentive to book Flex, if you're definitely travelling, so the mix of J and Choice will return similar SC's to consistently booking Flex.