Transformer Design: How do I set an N1/N2 ratio by means of selecting winding inductances?
It sounds like you're dealing with a SPICE simulator.

In SPICE, you define a transformer as a coupled inductor, with the following ratio:

\$\dfrac{N_1}{N_2} = \sqrt{\dfrac{L_1}{L_2}}\$

There's also a coupling factor K that comes into play.

The magnetizing inductance of a transformer is the inductance you measure across the primary winding with the secondaries open-circuit (floating). It's a function of the core material and geometry, air gap and number of turns.

Your \$L_1\$ and \$L_m\$ should be one and the same.