I'm trying to calculate the f/stop change of attaching a x2 teleconverter to a 80mm f/2.8 lens. In this instance, the teleconverter attaches to the front of the lens, and has a larger opening than the original 80mm lens.

The original lens would have a lens diameter 80mm/2.8 which is about 30mm.

The new lens seems to be about 40mm, and it's a x2 teleconverter, so the new f/stop should be 160mm / 40mm = 4.

All the documentation I found suggested that this is wrong and since it's a x2 teleconverter the f/stop should also be doubled.

Where's my mistake here?

Edit: turns out you can't trust numbers on off-brand teleconverters, it's actually a 1.4 when measured. still the question stands.