When using the Born-Haber Cycle for Na2S, do I double the ionization energy of Na since there are two Na ions for every S ion in Na2S?

Question

Also, the problem asked me to find the change in energy for S- (g) + e- => S2- (g). Assuming that you do double the ionization energy, I got an answer of about 550 kJ/mol give or take 50 kJ/mol. Is this value in the ball park or am I totally wrong?

Thanks for the help in advance!