Appreciate advice on how to avoid undercut. I practised on some plate first to get the settings then went up 1 notch on voltage and a bit on wire speed as I figured a 90ish deg fillet with 2 pieces of metal of similar thickness to the plate would need the extra amps.
Avoiding undercut will be a balance between reducing power, increasing wire speed, or reducing the speed of travel. Basically speaking there is insufficient filler being melted into the weld for the heat being introduced.