If I've paid discounted J and it's a 777 I'm totally going to MFU to F. In the good old days when we had a 747 on our route I was happy with the UD especially if I'd MFU'd from WT+. I don't really care about the value for miles J to F as opposed to WT+ to J. Actually it's become a moot point as the WT+ to J availability is terrible these days anyway from NA with all these CC miles promos from BA.