Next step remove handle, and mark centre of pot ( bottom side of course ) then scribe a mark from left to right .
From your centre line dot punch 60mm either side being 120mm apart .... drill these out with a 18mm drill bit
Next - If you don't have a lathe, get a compass and set it @ 39mm then draw a circle being 78mm, this will need to be cut out fairly accurately to line up with your 18mm holes
If you have a lathe, clamp the pot internaly and centre it up on centre point on pot ( I used shims )