Well done on the trade, I can see why you would want to trade in the deferred positions, but just be careful you don't get taken apart on the spreads.
You can quite safely trade FTSE positions up to the day of expiry, and rolling a position should only cost you £8.00 maximum (probably cheaper on IB).
Just for examples sake, Jun FTSE has traded c44600 lots, Dec 103 lots. The last thing you need is a trade that you can't get out of because nobody wants to play ball with you 🙂
.. but at the end of the day, each to their own!
2. The dec contract has the highest price so it will hit 3950 even if the ftse only rose to 3940.
Buy the same thought process, dec won't hit 3900 even though the nearby does.