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Try printing that 2 wall square I attached. The thinkness of the wall should be 1.2mm and 3mm tall.. This is how I find the proper extrude multiplier.
Never used a delta but I keep the speed to a max of 45mmS When I see a lot of stuff stuck to the nozzle it is normally because I don't have the starting height set correctly. once you get a bugger on there they keep on getting stuck.. PETG is very sticky. Also slow your speed when not printing.. axis movers.. default X on my MG was 300mmS droping to 200 really reduced strings and the nose snot you are talking about.