Question about welding thicker to thinner

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I am new to welding.  I am running a millermatic 180 Mig with argon mix and i believe .030 wire.  I am welding a seat bracket as well as two solid steel slugs to a motorcycle frame ('79 xs650 hard tail).  I understand keeping the puddle on the thicker metal and dragging the puddle down to the thinner.  My question is related to the settings on the machine.The first scenario im faced with is welding the seat mount To the backbone of the frame.  The PO when hardtailing welded a plate bw the two attach points of the hardtail to the backbone/down tube.  The plate thickness is 16 gauge and the down tube is 14 gauge. My miller is capable of 5/16 at voltage 10. About where should my voltage be/starting point to fine tune.Second scenario im looking at is welding two solid slugs about an inch in diameter To the frame tubing of the hardtail which is about 11 gauge or 1/8 in tubing Again what is the correct/starting point to set the voltage, also with the matic the wirespeed sets itself to the voltage but can be used as seperate settings.  Would somthing like slowing down wire speed slightly while keeping the heat up help in these situations?Any suggestions, experiences, knowledge is appreciated.Sent from my iPhone using Tapatalk