I think that you may have answered your own question there. However, if welding outside and it's pretty breezy then you could turn the pressure up to full but still not get enough shielding. I would look at some local wind break / sheltering of the weld before wasting too much gas. 10L/min should be enough if the weld is sheltered. Also, is the weld area clean? There may be some surface contamination there too that is carbonising and turning into 'soot'. Hopefully your gear is in good condition too with a clear shroud and diffuser?