How does the pressure change in the dimerization of nitrogen dioxide? say we have a reaction N2O4 reversible 2NO2

for this reaction,if initial number of moles of N2O4 is 'a' can we write

t = 0 a -

at equilbrium a-x 2x

similarly can we do the same for pressure? i.e.
if initial pressure is p,then can we say?

t=0           p -

at equilbrium p-y 2y

if yes why? and is x=y,i.e is the number of moles of x dissociated equal to decrease in pressure of reactant?
and is this method valid for all reversible reaction irrespective of the stoichiometric coefficient of reactants and products
If you assume ideal gas in a constant volume this works, but x is not y. 'a' and 'p' are related via the ideal gas law(a = n): P = n * R*T/V . 

Take in mind that the pressure actually increases as the reaction progesses as more particles are formed.