Bleeding the coolant system on a 944 is tedious to say the least. And, in my opinion, something that should not be overdone as it can develop air bubbles that will cause temps to spike very high, very fast. You often have to jack the car from side to side, shake it, etc to get all the air pockets out.
I posted a while back about an emergency bleed and fill with anti-freeze so that the all water system would not freeze up. I did a rush job and tested the coolant and it was only good to about 20 degrees. Not good enough here in Denver.
So, I need to get a bunch more antifreeze in the system but did not want to do do it normally. I figured if I cracked the radiator drain just enough for fluid to flow out I could refill with undiluted antifreeze without getting any air in the system. It worked!
I too off the reservoir cap, cracked the drain (I just cracked it, I did not fully remove the drain as it would get out too fast) and let the car warm up. Probably took about 15 minutes to get a gallon of antifreeze in. I just watched as the level in the reservoir slowly went down, and kept adding a bit of antifreeze. Simple.
Image thanks to Porsche AG website.