12 oz is 1.5 cups - I think I've gotten more than that. Might help to squeeze lower rad hose or put a bit of air pressure in the system.

But maybe nothing is wrong - When uncertain about the fluid I have measured the amount I had to add in order to refill the system. With the 850 I filled it and it took less than the specified amount. Squeezing lower hose helped get more in. After a day or so (warming up engine, driving, and letting it cool thoroughly), an air pocket evidently floated up and I was able to add the rest. Since the correct amount went in, I decided everything was ok. This has happened twice over the years.

Alternatively you could drain the system (collecting coolant for reuse) and measure the volume then.