If it were me 4qts is plenty to fire it up with. As for total capacity will depend on what pan cooler and lines you use. Considering there is likely 1.5ish in the converter you wont need a ton more for sure. Here is what ATI has to say. As for ultimate level I never run them completely full in the shop as you will likely not get enough heat in it to get an accurate read. Although engaging the brake will help put some heat in it.

http://www.atiracing.com/instructions/Trans-Converter-Installation.pdf