I think you will definitely be using HV mode on the highways, which should give you over 300 miles. I frequently drive the I5 and I405, pretty level roads, which do conserve much of the EV range, and save EV range for the city. The HV being a hybrid mode does try to conserve charge back to the battery whenever possible, eg on your downhills. It is hard to give very exact estimates of charge usage,however, for reasons below:
BY the way, have you gotten a hold of the user's manual yet? There there is a very complete description of the Clarity operations (not to be confused with the more limited info in theUser Guide) which is freely available at owners.honda.com. In brief, the three driving modes: ECO, normal, and SPORT, which for the most part merely map the same position of the accelerator pedal into lower, medium, and higher acceleration requests, respectively. There are the three running modes: EV, HV, HV charge, which direct the energy strategies towards primarily battery usage, hybrid usage (primarily engine usage with concern to save charge), engine usage (with concern to increase battery charge up to 58%).
At high accelerations any of the driving modes will become HV even if you started out in EV, to take advantage of combining the engine power with battery power.
And now to reveal a secret that your manual never quite mentions: Clarity knows exactly how much charge you put in the battery yesterday, but it does not know exactly how many miles those charges will take you today! It does not know: where you are going, how many people and how much cargo you will be hauling, how much heat or air-conditioning you or your guests feel the need to use, how loud you like your audio, whether there will be more up hills than down hills and how steep the slopes it will encounter, how good the roads are, whether you will be facing head winds or tail winds, whether you will be needing the defroster or windshield wipers, whether you are an aggressive driver today or more laid back and constant, etc.
It seems to estimate EV mileage based on what you did yesterday. So, if you want to see lots of miles on the estimated displayed EV mileage range tomorrow, remember to drive today in ECO mode, shut off the audio, and enjoy a serene quiet ride.