Sospan,
I have a plastic rod with a mark at 50% to measure on my CX. I agree a tab would help.
Robrhi,
I f you can fill the car on a slope with the front at the high point. Ensure the heater is on full heat and keep blipping the throttle & topping up.
The heater matrix are a high point on the circuit and it maybe you haven't got coolant into it.
