It's not a dumb question at all.

What you have to realize is that there is only two relationships that reallly count. One is the decalage angle and the other is the thrust line angle to the wing. Yeah, we just added another scoop to the mix...

The FRL or center line or whatever you want to call it is strictly an imaginary reference line it means absolutely NOTHING other than as an arbitrary reference line that the designer uses to set the other angles and parts to. Because of that there is no way to determine what it truly is.

However, let's consider that the human animal is basically lazy. Being that way I know that when I design my models I'll set the formers to sit at 90 degrees to the FRL for the most part unless it's a special case like a cabin former that sets the landing gear angle or some other special and obvious case. SOMETIMES, and don't rely on this, the upper edge of the main side panel will be drawn parallel to the FRL but there's absolutely no guarantee of that but if that line is 90 degrees to the majority of the formers you can pretty much rely on that being the case. If it's an aerobatic design or a super sport model intende as being close to aerobatic then you can expect the wing's airfoil angle to be very close to parallel to the FRL and the stab along with it. Often such models don't show more than a degree or so of decalage at most and are often 0-0. Such models typically also have the FRL passing through the center of the rear of the engine or motor although the front may be depressed slightly with downthrust. More oddly shaped designs won't follow any of these suggestions with any degree of reliability.

The bottom line is that if you need an FRL for some reason just draw your own in. It'll be YOUR reference and that's all you need.

To illustrate how confusing it can be I've seen old time Vintage RC models where the stab is set to 0 or even a couple of degree positive, the wing is set at a large positive incidence and the engine is set with only a little downthrust. But in reality the model set like this will fly very tail high so the wing sits at it's preferred angle of attack, the tail will be negative at that point and the engine will be working with a bunch of downthrust. The FRL in flight will be at a very tail high angle. In effect the plan angles the wing upwards to promote a larger downthrust angle. A VERY arbitrary reference line that has nothing to do with the reality of flying.