What I would like to add will be of little help until you find the real answer.
Drive long enough so you are not too tired. Tired is a fast way to end up dead.
You will not be able to figure this out until you try to work it. If you plug in XX Miles Per Day, you can count on it always being wrong.
We drive (DW drives most in the day and I navigate) as long as we are comfortable. This has varied between 220 miles in the snow and 1100 miles of flat but easy road. We have a coach that is VERY pleasant to drive. There are many times when we are not at all close to the planned End Of Day, and we just find someplace to stop. Other times, we seem to get to the EOD by mid-morning. So, that is when the navigator starts looking for another EOD spot.
Matt