I don't run the flexbone, but I run a run-first spread option attack. It's been my experience that the read guy is (almost?) always the last man in a 3 point stance. In a 4-3/most 3-4s it's the DE. In a 3-4 predator it's the DE or OLB, depending on which what you're running. Against the 2-4-5, it's a DT. Against a 1-5-5, it's the NT. I don't run options (being a spread team and not flexbone) against 5-2 fronts, but on some 5-2 plays, all 5 linemen are down and in some plays, the DEs stand up. I'd bet that when they're all down the read guy is the DE, but when the 2 ends are standing, it's like a DT.