30%w/w ammonia is about the solubility limit at room temperature, meaning 1atm ammonia partial pressure. If air is present before the volume is constrained, already 1+1atm absolute pressure is expected.
But this depends already on the presence of water before or after the volume is constrained. Dilution would reduce the ammonia partial pressure. By the way, I wouldn't trust a linear formula in this case, because the strong proportion of ammonia molecules in the solution lets them interact. This needs an experimental curve.
What can still happen is that the heat from the reaction with the acid reduces the solubility more efficiently than it reduces the proportion of ammonia. Then, the pressure may increase. This needs a numerical verification, based on the enthalpies of formation of HCl, NH3 and NH4Cl, all in water solution. The big ammonia concentration changes its heat of formation in solution, but I would just start with the heat of formation in dilute solution, hoping that the result is already clear enough.