In terms of road miles, traveling the most efficient route between Dallas, OR and Sebastapol, CA, the mid-point in the journey is about five miles south of Yreka, CA. If you stop in Yreka at meal time, consider some home cooking from Grandma's Table.
Four Corners, USA
The halfway point between Dallas, Texas and Warren, Ohio is in Centerville, Tennessee.
Washburn, TX is the halfway point.
Powderlick, OH.
Amarillo, TX
Dallas
Nashville, TN
1/2 is the halfway point
A rough estimate would be Amarillo, Texas.
Waco!
Turrell, AR
Cuervo, NM