I assume you weren't headed to San Antonio, so it should be 75 through most of your trip right? I thought that Texas was also pretty flat... So you should* be ok with going 75 and getting pretty close to the range, unless you were blasting the AC? Depending on how far you are going, I tend to give myself about a 30 mile buffer (unless traveling up mountains) and watch it closely. If it drops down to 10 miles of a buffer (and I am not pretty close to my destination (sub 30 miles left to go) then I would drop my speed by 5-10MPH and that would more than compensate for it. That is just me, and what I felt comfortable with, and note that going from VA to FL I never actually breached by 10 mile buffer limit.
I have learned that a lot of this comes down to driving habits and learning to drive the car in a more efficient manner. Cruise control is your friend! Acceleration should attempt to be limited to under 40kW as much as possible. Attempt to only use regen and use the breaks as little as possible. Also the more you coast vice regen the better range you will get. If you set your AC to LO, you will eat a big hit in your range... Just more subtle temperature like 68-72 should be comfortable (for most people) and will help a lot with not killing range. In the winter try to limit your use of the heater as much as possible and use the seat warmers. If you do most of these things you will find yourself hitting and even exceeding the rated range all the time.
My current lifetime average is 311w/mi if you take away the first 2000 miles of driving I did I am at 305w/mi. What keeps me from being sub 300 is that my daily commute is quite hilly. I get to work most days in the 320w/mi range, and then try to get it lowered back down to as close to 300 on the way home. This is with me going 70-75MPH, with 2 or 3 other passengers.
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