Does it decide what duster you get when you are talking to ulysses face to face or when the end cutscene plays?

Depends on your rep with the main factions. For example if you're going with Indie Vegas you need wild child ,at least, for the NCR, The Strip, and the Legion for the Indie Duster.

I meant when does it check for the fame and infamy values...

when you decide to bomb or not to bomb i believe.....

So if you were to bomb them it would check immediately after the infamy is gained...

So to answer question no it doesn't decide which duster you get when you talk to Ulysses face to face.

Not really, I heard some people still got the NCR duster even after bombing the NCR. It's because their NCR rep was the highest out all the main factions.

so as long as i dont increase the strip, followers,brotherhood, or legion i will get NCR duster...


EDIT: when does it check for the fame/infamy

Followers and the BoS ain't a factor when it comes to deciding which duster you get. You can increase you strip fame just because if you're idolised with the NCR it trumps all the other Dusters, not sure when it comes to legion duster though. As I said the main factions that matter ,when it comes to getting your duster, are the NCR, the Legion, and The Strip.

Edit:It decides the first time you talk to Ulysses though ED-E. Each of the four main factions gets a different dialogue in that scene.

ive read that someone got the inde. duster by having BOS and FOTA fame

I heard that theory was proven wrong, though I could wrong about that. To get the Indie Duster you have to be netural with the NCR, the Legion, and The Strip or at least have wild child rep with the previous factions I've mentioned.