This much! This image shows the maximum amount of ‘spill’ (flame extending beyond the edge of the base plate) that you want, and if you needed a number to work with I would estimate it to be 7mm of spill all round. If you allow the spill further up the walls of the Bacchi you can expect to warp the Bacchi sooner or later.
Unfortunately the town gas supply, in London (UK) at least, doesn’t burn anywhere near as hot as the propane/butane mix that portable camping gas bottles provide. So if you are using the Bacchi on your town gas supply you will find you need to push the spill to the upper limit, as shown below. This amount of flame spill gives a ready to pour time of about 6’10”.
When you use your Bacchi on the much hotter mix that camping gas brings you will find the opposite is true, and you will have to turn the flame right down (well inside the perimeter of the base plate) otherwise the Bacchi will whistle well before the 6′ mark and the water in the top chamber will be too cold for correct extraction.
One final point, which should be obvious, it is strongly advised that you use the Bacchi out of the wind. If outdoors a windshield around your gas burner makes life a lot easier.
