Don't overthink it. Timing is hard to judge, except in retrospect. The end of a conversation isn't always clearly predefined. Just go for it, lest the opportunity pass.
You could've gotten her number immediately after she waved at you and told you where there was a power plug. "Hey, I'm gonna call you next time I'm looking for a power plug. *hand your phone over to her*" Then converse as usual. Sociable girls give their numbers out to a lot of people, you don't have to be Johnny Suave to get it without hurting the interaction. As long as you show your confidence, any time is the right time.
Don't be afraid to fuck up. You pretty much can't fuck up by asking for a phone number. If she doesn't give it to you, you never had anything anyway.
For example, when she's leaving,
Her: "Goodbye."
You: "Hey, one last thing before I go. Give me your phone number."
You could've gotten her number immediately after she waved at you and told you where there was a power plug. "Hey, I'm gonna call you next time I'm looking for a power plug. *hand your phone over to her*" Then converse as usual. Sociable girls give their numbers out to a lot of people, you don't have to be Johnny Suave to get it without hurting the interaction. As long as you show your confidence, any time is the right time.
Don't be afraid to fuck up. You pretty much can't fuck up by asking for a phone number. If she doesn't give it to you, you never had anything anyway.
For example, when she's leaving,
Her: "Goodbye."
You: "Hey, one last thing before I go. Give me your phone number."