LOL... I've had a lot of "fun" trying this. I ended up running the engine at fast idle until hot, then shutting down and wait until it cools. Then fill the bottle until it reaches the full mark. Then repeat this until the level in the bottle remains reasonably high (between add and full). I know I have enough when 2 things happen.
1. Coolant flows out of the return and out back into the bottle when the engine idles.
2. The electric fans kicks in when the needle gets slightly past half way.
Bottom line is my '88 runs MUCH cooler now and I don't have to run the a/c as much (it turns on the electric fan).