I think I read in the owner's manual that the fuel tank capacity is 17 us gallons. When the little red pump light on the instrument panel comes on and the remaining miles left is around 20 miles, I fill the tank. But the gas pump always clicks off at about 12 or 13 gallons.
My previous vehicles at the same station would always fill to within about 2 gallons of capacity. I always try to set the pump nozzle at the slowest flow rate.
I'm wondering if this is normal procedure for the RX hybrid.

There's at least one major thread on this subject on this forum. The gas tank is not even close to empty when the lights/warnings/indicators come on. In my case, I did 853 kms (530 miles for you non-metric folk) and there were still 12 liters of gas in the tank (3.2 US gallons)

3.2g remaining is also what I've seen on my '17 RX 350 ICE, consistently. When the get gas light, 0 miles remaining, and Nav display changes to showing just a list of nearby gas stations I've only consumed 16g ('17 RX 350 has a 19.2g tank).