Perfectly put. I regard myself as far more advanced than any of my friends, but a total amateur compared with you boys and girls here. After too many years flying for BA, I assume that anything that can go wrong will do exactly that. When one has a tight connection - that is the day when the gates are so far apart that a Marathon Runner would be challenged. It is always - always the silly little puddle hops to the place from when the major flights should depart that always lets you down. That's one thing when it is all ticketed together but if it is on two separate bookings you are staring into a disaster.A friend of mine had a cancellation on a flight from CDG to AMS which was connecting with a flight to Brazil. They missed the flight to Brazil and the next direct flight was three days later. They were offered other flights which would have meant 27 hours of flights and connection which ate well into the short time that they had. This was KLM - but it could be any airline

Dear OP - you might be fine, but my threshold of anxiety would be such that I would never connect the same day on a separate ticket. There is too much at stake and you are not protected in such circumstances. I would go up the day before and sleep over at ORD. The JL flight is a 12.40 so that gives you fair time. Were there to be bad weather, or anything else you would be much better placed. Ultimately it is up to you, but missing your flight the same day. OK you'd have a night to pay in a hotel, but this is nothing compared with missing that flight to Japan.