Assuming you are using MIG since there is no amp control, just wire feed speed. The knob on most hobby welders only has settings from 1 thru 10, so to estimate wire feed speed you would need to cut the wire flush with the contact tip, pull the trigger for 5 seconds, measure the inches of wire past the contact tip, then multiply by 12 to get the number of inches per minute. Once you have the wire feed speed, you can estimate amps from the chart below:
However, the shade selection is really a matter of comfort as others have stated. Start with a shade setting of 10 and adjust from there. If it's too dark, go lighter. If you start experiencing eye discomfort after welding all day, go darker.