Your trans might not have been overfull. Note: Being overfull doesn't cause problems other than fluid running out the vent so dont panic. Its a "urban myth" that overfull fluid will damage a trans.
The valve body has "fluid passage exhaust ports" and there are spinning componants in side the trans. This fluid jetting into the fluid in the pan makes waves. The high point of these waves can make fluid come out even when the fluid is low.
Check procedure is to overfill then let drain till fluid coming out is no longer a steady stream. When it first starts "gurgling" then plug it.
Note: Also if I recall correctly then 30C is the correct check temp. I dont have time to look it up to verify right now though.