Torque in a 3-phase delta connected induction motor
How would I write it as N2/N1? Would it be 1/0.90?


\$0.90:1\$ as a ratio is the equivalent of \$ \frac{0.9}{1} = 0.9 \$.


  [...] the torque value I got was 0.6Nm. I'm not sure if it sounds about right or not.


A torque of 0.6 Nm could be a correct answer as I have no idea what you have for parameters. You might want to edit your question to include the values you are using. Additionally, you could include your attempted calculation.

Consider the following chart for reference:


1. Electric Motors - Power and Torque vs Speed