Hello everyone. Thank you for the replies. I think her vet was doing the 2mg/Kg. Since I'm in Mexico, everything is in kilograms.
I tried looking for the tablet form so that I could be sure to give her the 1mg/lb dose, but I couldn't find the tablets anywhere. I tried two of the large chain pharmacies and they only have liquid Benadryl. I even tried the Walmart pharmacy and it was the same thing. I have no idea why they don't carry the tablet form.
I was thinking of trying the 3ml and see if that works. It scares me, but the lower dose didn't calm her down. I don't really want to try giving her anything stronger. On the flight she doesn't bark or anything. She just pants the entire time once they start pressurizing the cabin. I wanted to just take the edge off of her stress. I'm going to try next week to give her maybe 2.5ml and see if that does anything. But first I'm going to find her vet's new office. They just moved yesterday to a new place. I have the address but I want to make sure I know where it is in case something goes wrong and I need to rush her to the vet.
Thank you everyone for your help.