I've found 15 CFM on the regulator's flow gauge to be plenty sufficient with my Miller 175 and C25 gas. Typically, I adjust a welder's flow down to about 10CGM on the gauge: This leads to instant ugly welds with huge voids in them: they look like rice krispies.
Then I simply bump up slowly until the finished welds surface is passable, and add 3-5 CFM on the reg's adjustment for a safety margin. It's like setting timing the redneck way: Advance until you hear pinging, back off slowly until it goes away, and then back off another hair in case changing conditions or small amounts of knock you can't hear are still there.
On windier days or with a fan running you'll need to bump up the gas's flow rate quite a bit, or switch to flux-core wire.
Something no one's touched on so far is wire diameter. I assume that for everything 1/8" and thicker everyone's been using .030 diameter wire in their MIG. If you've only ever welded with .030 wire on sheetmetal, pick up a spool of .023 wire before your next sheetmetal job, along with the matching tip for the MIG gun. You will be astonished how easy it is to run a consistent stack-o-dimes bead on exhaust tubing now. Burn through is about 50% harder to achieve.
You can even weld the paper-thin sheetmetal on imports with .023 without spending half your time filling holes: (This was my 12" bed shortening project on my Toy):
Millermatic 175 from cyberweld.com (highly recommended): $600ish
No more dragging tailgate on hillclimbs:
PRICELESS 
Oh, and in case your wondering, that Century welder I borrowed worked fine for this, but on anything 1/8" or thicker, it is HURTING. Experience with it is part of the reason I cherish my 220V MIG now.
