No need to run the car and worry about the converter being full.
You shoudl do it when it is cool.
If it is already running, just loosen the pan and let the fluid drain out until it is level with the top edge of the pan. Tighten the pan up, mark the dipstick at that level.
The proper fluid level is at the top of the pan, but it should be at this level running, warm and in neutral.
So once you mark the dipstick using my method, you will start it up, add fluid so that warm, running it is at the same level.