Meters have different settings but a simple guidline is to start at the highest setting on AC amps. My digital meter goes up to 10 amps. Make sure you use the right meter jacks for amps or you could blow the internal fuse or worse.
Most non-high output preheat lamp ballasts are between 200 and 430 ma with a starting current a little higher so if your meter can read at least up to a couple amps your pretty safe.
Once meter is set up, connect one meter lead to lamp end and the other to ballast wire. (Power is off) The other ballast wire goes to other side of lamp.
Double check connections and meter settings and turn on lamp and observe what the meter says. Once you make note of the readings, repeat same procedure with 18w sox lamp and compare currents. I'd like to hear results
As for the 20w ballasts, when you put two reacter ballasts in series, it cuts the current in half. They are rated at .35 amp and when I put them in series, I ran a 6 watt lamp at .17 amp according to my meter.