What is the single most important determinant in looking at 6 or 12 months forex rates? -

It depends. Keep in mind forex rates are RELATIVE values between TWO currencies . . it is not an isolated, one-dimensional concept. Sometimes the largest determination is the relative interest rates of their central bank (e.g. the Fed, ECB, B of E, RBA), sometimes how well its economy is going, political future, trade future. The list is endless.