when replacing my trans. drain plug (stripped therefore leaking) I drained the fluid from the pan. I followed the normal procedure for re-filling and checking the level. After running through the gears a few times the level was a bit on the low side so I added a little less than half a quart and now the level seems a bit on the high side. So I guess I'm having a bit of trouble getting an accurate reading. How much fluid is generally purged from the trans. when the plug is removed and drained?

thanks