well i think unless you could directly measure torque, you need to experiment with your motor first...
when i say experiment you need to carefully plot out the torque vs current graph. this need to be done at various speed so that you really could understand the effect of speed and torque toward the current.
if its linear or curve that you think you seen it in some math text book or you fine with some error then you could make a really nice formula.... of if its somewhat irragular then you need to make a look up table.... if you dont know what it is look it up (pun intended).... ok once you have the formula/look up table then you could use it to make the correction.