A: Hi DC, The Chimney Balloon should be a little springy (not drum tight) when inflated. If it is inflated enough to seal and hold in place, then it is inflated enough.
I know the temptation is to blow it as tight as possible, but that is not necessary. If you use an airbed inflation pump to inflate the balloon sometimes it helps to remove the pump just before the balloon is fully inflated and top the inflation off by blowing into the inflation tube. That will give you a feel for how much pressure is in the balloon.
I never recommend to use an air compressor to inflate a Chimney Balloon. They build pressure in the balloon too quickly at the end of the inflation, and it can easily blow out the balloon or the seals on the balloon before you have time to stop it. - Jason