Thi,
Since you replaced the new oil with synthetic, I suggest you to do flush instead than just drain the old one out, it's not good to have a mixed in there. To get the job done right, you need to have a machine that can do a flushing job. The reason that you are not sure, how much oil need to be filled up because you didn't get all the old one out from a draining.
One more thing to keep in mind, if you have more transfuild that it should be, you will damage your transmittion in the future another word is called "flood"
How to know your transmittion oil is new after replacing new one, the fluid should be "pink" as virgin. After 1000 miles drive should still be PINK.... I REALLY MEANT PINK!!!!