Sparkler send off

Our sparkler send off is at 9:30, does anyone have an idea of what time we should have people sent outside to start lining up? Our guest count is 80 so I'm guessing we will have anywhere from 65-70 guests still there to do the sendoff. I should also add the reception is upstairs at Our venue so guests will need time to make it downstairs. I'm thinking of having the DJ make an announcement for people to start heading outside and lining up at 9:15 then starting Our last dance as a couple at 9:25 so they should be lit up and ready for us by 9:30. But idk.