Hey scott, I guess I should be careful with my verbage. I actually don't know from experience that it is under there. I was told by someone who has had a Rover (Defender) for a lot longer than what I've had mine. I was told there there is a fill hole on top or around the ATF pan. I know there is because I overfilled from the dipstick and it came out this hole. I never really did get up under there to check it out to find the exact spot, I just stopped filling. It was great as the ATF came out on the first couple of trips I was a disco with a smoke screen as the fluid would drip out this hole and onto the exhaust "Y" pipes.
In my opinion and if I had the right tools I would fill from the pan because you know when you are getting full. I think there was a half to 3/4 quart on my garage floor before I realized what was going on.
After driving it and burning off the excess fluid I would check the fluid for about a week to make sure it stayed at the correct level. It has.
Sorry I can't be of more help
Peter