Dew point in wall – GreenBuildingAdvisor

In this article by Martin Holladay from 2017 https://www.finehomebuilding.com/project-guides/insulation/exterior-rigid-foam-thin (see attached image), he shows a diagram with R19 batt and min R 11.25 outside with the dew point landing in the exterior insulation. However, my calc for temp at back of sheathing puts the dew point inside the wall assuming R12 outside. Am I getting this wrong? Can someone check my math?

70 deg  – [45 delta x 19/31] = 42.42 deg at back of sheathing.

Just trying to wrap my head around the calcs and concepts and want to make sure I’m not misunderstanding!

Thanks

Brad