It depends when you triggered the REx. Leave it to the car and you'll use all bar 6% after about 120 miles then the REx will trigger. At 70mph you'll be about neutral at that speed, neither charging or discharging except on hills, strong winds, rain and cold.
You can choose to trigger the REx earlier and roughly keep whatever charge you have at that point, subject to REx range of about 65 miles before running out of petrol. Either way you'll be fine.
I have a 2018 i3s REX and done 30k miles - lots up and down the M40.
At 70mph in winter when it's wet and cold you won't get much above 3m/kWh .. so 90miles off the battery. You'll probably use most of the 9litres of petrol to do the other 65miles.
Best case in summer you'd get high 3s/kWh .. so nearer to 120miles.
Dropping your speed by 10mph will have a big impact.