If your commute is 38 miles then most of your journey will be in e-mode so you will get huge mpg. Let the battery drain and switch to ICE naturally then recharge when you get home. You only need to keep 20%+ charge if you're leaving it at the airport car park while on holiday as it uses the e-motor to start the ICE. No charge = no start!
Max range depends on a few things - ambient temp, driving style, opportunity to regen downhill, etc. I see around 35 miles range on full charge.
This engine configuration is not optimal for regular long journeys, definitely more suited for regular short journeys.
For my commute its perfect. I put a tank of fuel in March and there's just under half left now - I use the car most days almost exclusively on e-mode. I only charge at home, never in public.
The part about keeping 20% is incorrect. The battery is 13kwh but only 10kwh is available for everyday use. The system always keeps 3kwh in reserve so that you have full performance available even when the guauge reads 0%. This is why the engine is sometimes over reving at idle as it is charging the battery.
I've had mine for over 3 years and 65,000 miles now and left mine on 0% plenty of times and it has never failed to start.
Regarding the fuel consumption I've found it varies greatly and depends entirely whether you charge it up or not, how hard you drive it, the distance you cover and the temperature.
I've seen between 30 and 300mpg but generally get around 40-50mpg.
Without charging and with trips above 50 miles I get around 46mpg.
With charging and trips around 100 miles I can get 60mpg. Further than that and the charge generally runs out and it starts dropping down to late 40s.
Local trips with charging can be done electric only up to 23 miles ish in the summer and about 15-18 miles in the winter.
I got mine as a company car tax dodge. It is fun to drive with good performance but my old 2L diesel Superb was more economical.