Rather than math, I'ld look at a WWS or Recount, and compare total damage output. Relative to each other, Rune Strike generates 50% more threat than Frost Strike. Thus, Frost Strike needs to go 50% more damage to make it superiour for threat.
The advantage of looking at it from damage is you bypass GCD conflicts, KM procs, playstyle and/or spec variations. IMO, I tend to find FS beating RS, but it looks more like Runic Power starvation (IE, the player dumps all RP into FS, and RS isn't eligible to trigger). Adjusting your playstyle to allow that 20 RP 'floating' can significantly improve RS hits, and thus the generated threat. Note that FS is losing the 'can't be blocked/parry/dodged' in 3.2, which makes RS even better if you manage your RP accordingly.
I wish that there were 'combative' training dummies that did 1pt melee swings so we could more accurately simulate tanking, rather than only DPS.