Sounds about right to me.
Moreover, I was once told (by a fireman or gas worker — don't remember which) that when the gas concentration gets extremely high, the danger of an explosion is lower than with a moderate concentration, because with so much gas there may not be enough oxygen in the house to allow combustion. He said that for this reason, it can be very dangerous to open the windows to let the house “air out” — unless the procedure is done by professionals who know exactly what steps are required.
In other words, if you think your house has a gas leak, don't operate any kind of electrical switch, don't leave any windows or doors open, and get out right away.
Yep. upper explosive limit is about 16%.
Call the gas company. Touch nothing. Get out. Get away.