How do i find nichrome temperature
You can't. Sorry. 

The temperature of the wire depends not only on the electrical power on this wire, but also on the ambient conditions - i.e. how the heat is dissipated. This way, the temperature of the wire will not be constant in time, but will change depending on the air temperature and movement, materials that are in contact with the wire and so on. 

In order to make the temperature constant you will need to make the ambient conditions pretty stable, or to use temperature regulator that to regulate the electrical power on the wire in order to get the same temperature in different conditions.

As long as the theory behind such phenomenons is too complex, the calculations are actually possible, but very complex. In most cases, experimental way is much more reliable and useful. 

Simply use regulated power supply and change the wire current until the temperature becomes what you need. But always remember that in other conditions it will be different.