I have been welding for most of my life and I have absolutely no idea what settings you need with that sort of set-up.....Sorry.....
Try the middle.... then flat out cos I reckon it'll look like pigeon droppings... then lower it a bit.......

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Hang on... did you say you've had it for 5 years and you haven't figured out the settings on it yet?.........
There must be loads of people that have this sort of welder tho.... So you must be able to get some help....