If you have separate internal stems that need to be attached to the end stem, then you need to cut to the inside line.

The added bit that becomes the internal stem will be the same shape on the inside as that line. On the outside it will be the same shape as the outer line.

The bevel goes across the full width of the internal stem, so it starts where the added internal stem meets the end stem.

Internal stems are a very good idea. So are external stems.