Strictly speaking, the value of Q you use in the equation should be dimensionless, as you can only take the log of a number, not a physical quantity. For this purpose you express the pressures as dimensionless numbers, by dividing the pressure by the standard pressure P°. If P° = 1 atm, then P_{MeOH}/P° = 0.855, and so on, and Q_{p} = 4.90 x 10^{-3}. If you're working in Pa, you divide by the value of P° in Pa, so you should get the same number. Similarly for Q_{c}, you divide the concentrations by the standard concentration (1M). "Standard" means whatever is defined as standard in the definition of ΔG°.