How is Transformer's Current Rating calculated
For the development of a transformer a few steps are required.


Power rating


2.Voltage levels (primary and secondary)

3.Currents on both sides

4.Primary and secondary coils wire diameter/size

5.Iron Core area

6.Numbers of turns (primary and secondary)

First the size of the magnetic core has to be determined. From there the number of turns per volt.Next step is to calculate the windings for the required voltages. Then the wire gauge is to be calculated based upon the alowable current density. 

The way you calculate the N1/N2 is ok but you can not reduce the number of turns just like that since that depends on the properties of the magnetic core.

More in detail you can find a complete calculation in the mentioned link. There they explain all the calculations involved in designing a small transformer.

http://www.engineerexperiences.com/design-calculations.html