BikeCAD does not produce a miter template for the back end of the chainstays. I assume you are using hooded dropouts. If you were using socket dropouts, you would not need a template. If you were using plate style dropouts, you would slot the rear ends of the stays. It is for the hooded dropouts that I regret the lack of templates. BikeCAD will show the distance along the edge of the chainstays as shown below.

However, be aware that these dimensions E1 and E2 go to the unmitered ends of the stays. If you use the hidden line display, you can get a sense of how much the dropouts overlap the stays at this point. You can create a user dimension to get a more exact measurement.

As for the flat mount, you can select the Generic rear flat mount option as shown in the first image to show where your flat mount should be placed. Ultimately, when it comes to brazing this piece in place, I would recommend a fixture such as this one by Incepi: incepi.net/collections/disc-mount-fixtures/products/copy-of-post-mount-disc-brake-fixture.

For additional info, you could import your model into FreeCAD. As you can see below, hooded dropouts in FreeCAD will actually carve out the stays so you can reference this model for your miters.