My reasoning is always put back in what you take out. The proper way to do this is measure the fluid in the car when the ATF reaches 176 degrees F. Technically you need an a bosch hammer or infrared thermometer from HF or somewhere, but like I said, just put in what you took out and you should be fine. The entire system holds nearly double the capacity you removed, so as long as you didn't have a leak before. I would actually add an additional half quart to what came out originally. I believe there was a TSB from Porsche AG about this very issue. Here:
http://rennlist.com/forums/attachments/964-forum/415025d1264393213-checking-tiptronic-atf-levels-tip-fluid-tsb.pdf
This TSB is for the 964, and was released in 1995. I am unaware if the sight gauge was corrected on the 993 or not. .5 quarts overfill fill not damage the system regardless however.