Should I do my analysis using SPSS based on sample population or returned questionnaires?

#1
We recently participated in a survey in which the sample population was 640 students while the questionnaire return rate was 629. To capture the demographic information, the students were asked to specify their gender. All the 629 students who returned the questionnaires responded to this question. 319 indicated that they were male while 310 indicated that they were female. To find the percentage of male and female who participated in the survey, should I calculate the percentage out of 640 or out of 629?

i.e. should I do

(319÷629)×100%

or (319÷640)×100%

to find the percentage of male students who participate in the survey?
 
#2
If you calculate the percentage out of 640 you should specify the percentage of no-response (1.7%); if you use the value 629 you should specify that you compute the percentage on questionnaires responded. The difference anyway is really small.