No worries about it.

The cross sectional area of the front chamber varies depending on where abouts you measure it. The diaphram is a cone shape, so as you move in toward the dustcap the cross section area gets progressively smaller. 80cm^2 isn't really accurate either, seeing as it will be a range of values (cross section area at the surround will be bigger than cross section area nearer to the dustcap). I use this number though just because its somewhere around the middle of the possible values. It doesn't matter anyway for what you want to enter into Hornresp seeing as it doesn't predict response at the top cutoff accurately.

I've found in the past that spacer's between the driver and mounting plate are generally a waste of time when you do the measurements. Extra volume in the front chamber isn't needed when your doing a horn like this. I would forget about it, and mount the driver directly onto the mounting plate and whatever air that is trapped underneath the cone will be your front chamber volume.

Adrian