You would be correct, assuming the flap is completely rigid. Fortunately for this purpose, it isn't, and also fortunately, sanding a thick balsa sheet to a taper releases enough other stresses that it swamps the exceedingly low stress induced by sanding these two "out-of-plane" surfaces on a flat tabletop. So the "two rods" method, with or without a jig, is a perfectly satisfactory way of achieving the goal.
Brett
Yes, you can manage with 2 rods , but you will have to glue the t.e. rod temporarily to very centreline of t.e. Cutting a well-fitting groove in the c/l helps a lot with that.
If you want to have both rods against the table, you'll need 3 rods;
-1st t.e. rod with ø= (flap total thickness :2) + (final t.e. thickness : 2)
-2st t.e. rod with the ø=final t.e. thickness.
-And the front rod with same height/ø than flap l.e. thickness, of course.
That is naturally more complex and has a bigger risk of introducing an error to the flap, so in my opinion gluing the rod in flap c/l is better.
But even better, more stable and often with a better stiffness/weight ratio, is a built-up flap with 1/20..1/16" skins and some ribs inside.
Using same kind of highly complex calculations as above, it is very easy to sand or hot wire-cut a foam building jig with correct surface twist to produce a straight flap.
I'm just about to build such flaps for my new model, so I can document the process in here. But don't hold your breath, it'll take a while. L