Love [name_u]Dagny[/name_u]! [name_f]Evangeline[/name_f] [name_u]Dagny[/name_u] [name_u]Louise[/name_u] is in my top 3. 
I would love to meet a [name_u]Dagny[/name_u] [name_m]Taggart[/name_m]; I’m not familiar with [name_u]Atlas[/name_u] Shrugged myself, but it’s a super cool name, and I am a huge [name_u]Dagny[/name_u] fan.
I’d be hesitant to use [name_u]Dagny[/name_u] up front, because of comments I’ve heard from my family (all dagger this and dagger that, le sigh), but I do adore her and wouldn’t hesitate to use her as a middle.
I wouldn’t be concerned with the flow of [name_u]Dagny[/name_u] with your last name, although I would personally hesitate to use a name that’s the exact name of a character in a fairly popular film or book? Maybe [name_u]Dagny[/name_u] as a middle? I don’t want to encourage you to hide her away, lol, but that might take away all the concerns. I think something like [name_f]Abigail[/name_f] [name_u]Dagny[/name_u], [name_f]Eleanor[/name_f] [name_u]Dagny[/name_u], [name_f]Kaia[/name_f] [name_u]Dagny[/name_u], [name_f]Serena[/name_f] [name_u]Dagny[/name_u], [name_f]Helena[/name_f] [name_u]Dagny[/name_u], [name_f]Fiona[/name_f] [name_u]Dagny[/name_u], [name_u]Auden[/name_u] [name_u]Dagny[/name_u], [name_f]Magdalena[/name_f] [name_u]Dagny[/name_u], etc., all work super well.
Good luck!