After mortising the style, I hold up the rail to the style, so I can mark where they are going to meet.
At this mark I scribe the slanted edge of the styles and cut with a tenon saw.
This part is now offered up to the style to mark the position of the other shoulder. The width of the tenon is transfered directly from the mortising chisel, without further measuring.
And when everything has been executed with care this results in a nice fitting joint.