My understanding is that, as you say, the traction battery retains about 30% of its charge. There are good reasons for this but 30% does seem on the conservative side to me. I would prefer the app showed the percentage of usable charge; 100% when fully charged and 0% when there was no usable range left but it's not a big deal.
Same is true of the dash display; if I can't have/use the last 30%, then don't show it. True of the fuel tank too; if it's full to the top, I want to see all the bars in the green and for the number of bars to reduce proportionally until the tank is empty. Then I can be clear and know that if it gets to one remaining bar, I need to start looking for a petrol station. Toyota/Lexus conservatism is annoying here; as another thread has shown, owners have found that there is often still a significant amount of fuel left in reserve when the dash would have you believe that the car is in imminent danger of running out of fuel and being stranded.
Estimates on the dash display and the app cannot be calculated in the same way. My dash has been showing an EV range of between 71km and 73km for weeks now whereas the app has been gradually creeping up by 0.2 or so without a/c and 0.3 with a/c over the same period. However, to be honest, I just let the car get on with it; if the fuel looks low, I top it up, if the battery needs charging, I charge it - a bit like the olden days before we had these fancy apps! I know that if I drive gently, I get more mpg or battery miles, if I drive more aggressively my £ doesn't take me as far and I don't need an app (or a home smart meter) to tell me that.