What will be the trapezoidal angle of the bottom-layer routed trace to get the same impedance as the top-layer routed trace?

When the top-layer routed trace trapezoidal angle is less than 90 degrees, there could be two inputs for the layer stackup trapezoidal angle at the bottom layer.

As an example, in the following stack-up, the top-layer trapezoidal angle is 75 degrees.
 

At the bottom layer, there could be two inputs for the Sigrity PowerSI stack-up for the same trace:

1). 75 degrees for the bottom layer

2). 105 degrees for the bottom layer

This is because, at the bottom, the trace is upside down.

Currently, if you want the trace on the bottom layer to have the same impedance as that on the top layer, the angle of the bottom layer should be "180 minus top-layer angle", which is equal to 105 degrees, click here for more detail.