Lots of articles mention pressure, but don't specify the amount of it:
One study records the velocity of the spray to be within a range of 325 to a stunning 1950 cm/s. [...] Once the muscles around the reservoir squeeze the first amount of reactants through the valve into the reaction chamber, the resulting explosion causes the pressure to rise rapidly in the reaction chamber, forcing shut the one-way valve. The products of the reaction then exit the chamber with a pop and a puff, and the pressure inside the reaction chamber lowers again, falling below the pressure of the collection reservoir, which is still being squeezed by the reservoir muscles.
Calculating from the spray characteristics is beyond me.