Have I overfilled it?

Next time when do a filter and fluid change buy you one of these

http://www.summitracing.com/parts/MRG-4470/ and install it on the pan. In case you overfill it you can just pull the and drain plug and drain a little out opposed to removing a line. You might find a higher quality one than mister gasket however.

Installed one on my Dodge Ram and actually enjoy servicing the transmission now. lol The main reason I installed one is that it was recommended to change fluid every 15k miles because of the troublesome Dodge transmission I have. I have to adjust the bands more than anything.
Fortunately I have never had to drain any out but there is always chance that might happen.

On automatic transmissions it is better to be low on fluid than to much.