have an IR thermometer? do your thing with the iron and aim for a tip temperature of 350C. 63/37 rosin core solders pretty much all process around 350
the problem with low settings on the wlc100 is that you have poor thermal recovery. you process a joint at 350, the tip drops below process temperature, and it takes a while to heat up again. the problem with high settings is that it will happily get the tip much hotter than 350C, at which point you will problems with burning your joints and splatter from the solder (especially thin solders). when using an iron like this, you need to develop kind of a zen feeling for how hot your tip is and adjust the power to the tip accordingly so that you get 350C steady state. i was never any good at it, so it didn't work out for me. shrug. ymmv