Sounds like it's time for another primer on the oft-misunderstood F code:

First, as was said by someone earlier, the "less than 18 hours from report" has nothing to do the time between when ARCOS calls you and when the report time of the GS is. You can get an F code GS that reports 40+ hours from the time ARCOS calls you.

The F code is given when a rotation reports on a LC day at a time that you are otherwise not eligible for report. For example, today and tomorrow are VAC days with LC the day following. Today at 1200 you get called for a GS that reports in 2 days at 1600 (52 hours to report). Because you are coming off VAC you are not able to assigned anything before 1800 on your first LC day. So any GS that reports before 1800 would be awarded as an F code. This same thing applies coming off training (such as CQ) where you are not contactable at all and cannot be assigned anything prior to 18 hours after you resume LC.

Coming off a non-fly day (X day or assigned 30 hour rest), you cannot be assigned anything that reports before 10 hours after the start of LC, and anything between 10 and 18 hours after the start of LC must be on your schedule by 12 hours prior to the start of LC. So for example, today and tomorrow are X days with LC the following day. Today you get called for a GS that reports on your first LC day at 0600. This is an F code GS because you are otherwise not eligible to be assigned that rotation (note the word assigned, as in forced, as opposed to awarded via a YS). Once you hit the 12 hours prior to LC start point, then any GS rotation that reports within the first 18 hours of LC would be an F code since you are past the point they can assign you an early-report assignment.

Now, once you are actively on LC then the award/report time window starts to apply. If the first day of the rotation is a flight duty period then you need 10 hours from notification to report in order to be legal, so any such rotation would be an F code between 10 and 18 hours from award to report. If the first day of the rotation is DH-only, and thus not an FDP, then a reserve needs zero rest and can be awarded such rotation at or even past report time.

As for pay, an F code will pay the Block+DH+MU for the first duty period, or the duty period minimum of 2:00, whichever is greater, on top of reserve guarantee. I see a lot of people expecting 5:15 when they fly 1 leg ATL-BHM on the first day then lay over. (A 1-day rotation will pay at least 5:15). So be very careful and make sure you really understand the pay implications for acceping a GS that reports on a LC day. You also get no PB day for that day, and none at all if the entire rotation is over LC days. So it'd be a real bummer to blow G#1 for 2:00 extra on top of guarantee and work 2-4 days for it.

Remember that F#X = G#X. The coding is critical because the G code will never pay the first duty period above guarantee. Only the F code does that.

Bonus note: an OOBGS that reports on a LC day will always pay the first duty period above reserve guarantee no matter what because you are never eligible to be assigned that rotation regardless of the 18 hours from report guidelines above.