Continuing my suggestion from above:
If the subterfuge involved solely the creation of the badge, then the quality of the badge would only matter specifically against attempts to examine it to determine if it was forged. So if he flashed it at a bored security guard, the guard would likely glance at it and wave him through. If he showed it to a police officer securing a crime scene, he might look it over (roll against the quality of the badge, but only if the cop is looking for a reason to deny access) before waving him on. If he was caught standing over a body and tried to use the badge to get away, then it might require some sort of Deceit roll (modified by the quality of the badge), with a failure resulting in a call to the FBI office to confirm his identity and alibi (with bad results, given that there'd be no record). If he showed it to the guard in the lobby of an FBI building, the guard would say "That's a nice shiny badge you've got. Congratulations. Now scan your ID at the badge reader if you want the door open."
Better yet, instead of creating just the badge, the forger might forge a complete identity. This might include hacking a personnel database at the FBI so that the the quality of the forged identity could be used even in the case of the phone call or badge reader. This is a much more difficult and much more complete subterfuge. It might also be more dangerous, since the forged records might be noticed by a random sweep of the database, with the resulting inquiry following close behind.
Another alternative might be to impersonate a real agent. This would have some advantages (less need to hack computers, unless codes are needed to fool card readers), and also some disadvantages: "Oh, Bob, great to see you again. My, you've lost a good deal of weight! [Fiddles with holster.] And you look to be a good 4 inches taller than when we got together at the bar last Thursday. [Levels pistol.] Now tell me who you really are, dumb-shit!"
If this subterfuge is an intrinsic part of the character -- ie he's living a lie full-time -- then you should absolutely reflect this with one of the character's aspects. Maybe "I'm with the FBI ... honest!" This would probably require having a fully-fleshed false identity, and could generate compels and such to make the character's life more interesting.