That's what I did on my recent trip through the Columbia Gorge. I had first run into strong headwinds from Grand Junction CO to Green River UT and didn't slow down when I first saw the energy plot dropping. Later, I had to drive slowly at 50-55 in an 80 mph zone for many miles before I could speed up a bit when I knew from the plot I had it made. Rolled in with 3.5% (7 RM ) left. Lesson learned: when the energy plot shows something sapping energy faster than expected, slow down a bit right away until the plot stabilizes and I can figure out what is going on.
After that experience, the next day when I hit strong winds driving from Pendleton OR to The Dalles OR, I knew to slow down sooner (mostly 59 mph according to my notes). As I got closer and knew I would make it, I was able to speed up a bit and use much of my buffer. No problems, rolled in with 4% (8 RM). I could tell the wind was strong from the whitecaps on the river; from my sailplane days I knew that it takes at least 15 mph to make whitecaps, and these were BIG ones. Knowing what I was facing, I charged to a 25% buffer at The Dalles and was able to drive at the speed limit through the Columbia Gorge (in heavy rain) to Portland with no problems.
My rules:
1) If energy remaining is falling faster than expected slow down right away until usage stabilizes. When nearing the destination speed up again once it is clear that I will make it.
2) If I know I will face challenging conditions — such as wind, rain, cold or whatever — charge to a higher level to increase the buffer.