Norman testified that he and Jarman didn’t return to the building until after they got word that the motorcade was on Main. If you believe that the timestamps on the police radio transcripts are accurate (and what WC evangelist doesn’t) then that didn’t happen until 12:28.
The motorcade turned off of Main onto Houston at 12:29 but it turned onto Main from Harwood at 12:22 or 12:23.
12:20-21: "About to cross Live Oak" (They're still on Harwood, about three blocks north of Main)
12:22: "Escort drop back, go read slow speed now. Approaching Main." (Harwood and Main are eleven blocks from Dealey Plaza and the motorcade becomes slowed by the crowd)
12:24 (est.): "Just crossing Ervay Street" (Transcript doesn't specify a time; they're on Main, two blocks from Harwood)
12:25: "Crossing Lamar now." (On Main, four blocks from Dealey Plaza)
12:28: "Just crossing Market Street." (On Main, two blocks from Dealey Plaza)
Mr. BALL - How long did you stand there?
Mr. JARMAN - Well, until about 12:20, between 12:20 and 12:25.
Representative FORD - This was about what time?
Mr. JARMAN - That was about 12:25 or 12:28.
Representative FORD - You got off the fifth floor?
Mr. JARMAN - Yes, sir.