To actually answer the question, there's no real sweet spot.
Entering the highway at full charge doesn't leave you any headroom to absorb downhill or deceleration energy; I typically switch from EV to EV Later when I'm on the ramp, when I know I won't be stopping for the next few minutes as the ICE warms up (so to speak).
For best battery lifetime, it's probably 50% or below; the cells prefer not to be at 0% or 100%.
For efficiency, it doesn't much matter, just cycle all the way around to EV later at the bottom of the hills to reap the energy you just gained.
Of course, if you play the game like I do, and cycle around whenever it's above the set point on the (mostly) flat, you're using gas to charge the battery, which is probably a losing proposition unless you're going to use that energy (switch to EV or Auto) when you leave the highway.
Slowing from freeway speeds gets you a few percent, so slowing down for an interchange impels me to cycle the EV button while on the ramp.
I also drive in L, and have learned to feather the pedal to charge or discharge pretty much as I wish.