Hi, most aroid tubers (in fact all of the ones I routinely deal with) root at the top; the root plate surrounds the growing tip. Therefore anything between 2-6" will be fine, but usually you only need to plant deep if the plant is outside over winter or has any chance of drying out.
I think it's fair to say that yours is now, beyond a shadow of a doubt, going to flower!!! In my experience of growing them outside they often linger tantelisingly at this stage for a couple of days, but yours is probably opening right now. Also I think they tend to open up over night...
greenBen