You're trying to get the very core of your workpiece as close to 100 degrees C as possible, you certainly want it up to at least 93 degrees C.
This isn't easy and gets harder the thicker the workpiece, by doing the job outside in winter you're not really setting yourself up for success. The "1 hour per inch" is only a very rough rule of thumb for steaming, your 1 1/4" workpiece may well need two hours steaming even under ideal conditions, and you'll struggle with a 1 1/2 litre tank as the workpiece will cool right down every time you re-fill, even with an insulated box and topping up with boiling water you might be better aiming for 2.5 or 3.0 hours steaming.
Once the workpiece is bent around the former you need to dry it right out before cutting it loose, for example getting down from 25% to 10% moisture content in 1" stock will probably take about a week inside your home, longer in your shed. Don't rush this.
Forget steam bending tropical timbers or most softwoods (Yew is a bit of an exception to that rule), and personally I wouldn't bother steaming kilned timber. It's not that kilned wood is impossible to bend to typical windsor chair type radii, it's just that the failure rate can easily top 80%. Personally I'm not prepared to put all that time and effort into something that might only deliver one success out of five attempts. My experience has been that air dried timber is almost as good for steam bending as freshly cut timber, however I seem to recall reading that James Mursell says freshly felled is significantly better than air dried, so I guess there's still room for debate on this one.
Good luck!
