By "current-lead" this means leading as in the first condition you focus on. That is, you work out the welding current you will need, then set up the MIG welding machine to get it. Here is advice as a was directed to it. Thanks to to Miller, makers of welding machines I can very much commend - based in Appleton, Wisconsin, USA.
In this method you use the "general rule", which applies not just to MIG but also to TIG and SMA welding, that you will be wanting about 1Amp of welding current per thousandth of an inch of material thickness. Or you can think of it as 40Amps per millimetre if you want to think in metric terms.
from http://www.millerwelds.com/pdf/mig_handbook.pdf - illustrated explanation on pg4 Setting up a MIG machine - current-lead method (There is a voltage-lead method) Taking example of 1/8th-inch=0.125in (3mm) plateCurrent<->thickness rule -- 1Amp per 0.001inch of thickness (40A per mm) So looking for 125A Consider/survey/select wire size (diameter) options given their current range. Wire-size | Amp-range 0.8mm (0.030in) | 40-145A 0.9mm (0.035in) | 50-180A 1.2mm (0.045in) | 75-250A Each of these three wires accommodates the 125A needed Deduce from known wire feed speed <-> current relationship the wire feed speed needed, then set it with MIG machine's wire feed speed adjuster The w.f.s. <-> current relationships Wire-size | Recommendation 0.8mm (0.030in) | 2 inches-per-minute per Amp 0.9mm (0.035in) | 1.6 -"- 1.2mm (0.045in) | 1 -"- So for our 3mm (1/8th-inch) weld, would be Wire-size | calculation | w.f.s. 0.8mm (0.030in) | 2 * 125 | 250ipm 0.9mm (0.035in) | 1.6 * 125 | 200ipm 1.2mm (0.045in) | 1 * 125 | 125ipm Select voltage Low voltage -> wire "stubbing" Correct voltage -> smooth running (the "crackling" sound on dip-transfer) High voltage -> arc unstable, spatter