Calculate Gibbs energy of dinitrogen pentoxide using its vapor pressure and sublimation
First, here is a way to solve it that avoids calculating entropy:


Calculate the enthalpy from the given data.
Calculate the vapor pressure at 25 $^\circ$C. This is equal to the equilibrium constant for the process.
Calculate $\Delta G^\circ$ as -RT ln(K).


This way is efficient because you already set up the Clapeyron equation in the first step and you just use it again in the second. You could even solve 1. and 2. graphically, and then go from there.


  I got within a few significant figures when I used Tsublimation. 


You need to calculate $\Delta S^\circ$, i.e. the change in entropy when all species are at standard state. At the normal temperature of sublimation, $\ce{N2O5(g)}$ is at the standard state, so that is the temperature you need to use. The enthalpy is not concentration dependent, but the entropy and the Gibbs free energy are.


  I am not sure if I even solved the problem correctly[...]


You solved it correctly in that you got the correct answer. You did not solve it as the problem author intended, I suspect, because you had to look up the normal sublimation point (unless you used the Clapeyron equation to estimate it).