The difference between fthreat and fthreatscale is which type of threat you want to see.
fthreat will give you the threat relative to the tank, so if you are ranged it would mean you have till 130% of the tanks threat till you pull aggro.
fthreatscale will tell you your current threat as a total of 100%. What this means is when you hit 100% threat you will have aggro.
Peronsally I prefer using the fthreatscale option since it is easier to gauge threat if you are ONLY staying at range if you play a ranged DPS.
If you are having to move in and out of melee range on a target I would suggest using the fthreat value instead. The reason being is that fthreat will tell you how far ahead the tank you are so if you are past the point in which melee would pull aggro and you have to move into melee range you will pull aggro just by moving into melee range. I have done this on my warlock before
If you have any more questions just keep them coming and people will answer them.