I had to reread the rules closely to remember, haha. You don't determine the clock until the Gig is over. It's tucked under Advance Gigs: the Fallout clock has segments equal to the highest result of the Extraction Pool. So if you failed a Difficulty 4 Gig because your highest result is a 3, you'd get a 3-segment Fallout clock. Or if you succeeded with a 4, but you rolled a Threat, you'd get a 4-segment clock.
If you would apply the Fallout tag to a Gig that already has it, you add a Glitch instead. As written you can only ever create one Fallout card from a Gig, either from failing or from rolling Threat. But you could totally stack Fallout tags and create more of them instead of converting them to Glitches. I think it'd be a lot harder, but interesting!