Calculate wire temperature
Presumably, you want to predict the temperature at the wire surface. The problem you face is that it will vary drastically with conditions, e.g., is the wire supported in free air, straight or coiled, or in contact with materials that will conduct energy away from the wire, and so on.

There are various approximations for some common configurations that can yield a number that may or may not be close to the actual temperature in a practical situation. All depend on the same quantity, the energy input, which is easy to calculate. All make approximations of the various mechanisms that exist for carrying that energy away from the source, i.e., your piece of wire.

In most practical situations it's easier to think of the nichrome element as an energy source, and allow for ample safety margin in the temperature tolerance of the materials used in construction. You should be able to find examples of situations similar to yours on line; noting what element configurations worked in these will offer valuable insight to what you can expect in your design.

It is, of course, possible to analyze your exact situation much more closely, with hard numbers predicted, but you still need to verify the predictions with careful measurement. A good example is here: 

http://dspace.library.uu.nl/bitstream/handle/1874/32718/sdarticle.pdf?sequence=2

Depending on what you want to accomplish, it's likely to be easier to run some experiments and dial in the results that fit your design goals.