Easy. Dont use the settings.
Go "borrow" the bath room scale or the kitchen one. Weight the amount you need for your area. For example, AS (21-0-0) is 21% nitrogen by weight. So a 1lb of AS = 0.21lb of N. Let say you want to apply that rate (0.21lb of N/ksqft), then you will need 4.5lb of AS.
Once you have the 4.5lb then place it in the spreader and go to the lawn. Open to the lowest setting and walk around. Nothing coming out? then go to the next. Keep going up until there is a low flow. Keep the flow low and go around the yard east to west and north to south.
Problem with this approach - the spreader was empty half way thru the yard. This means it was too open and you applied double the rate (to the portion that you walked). Next time dont open it so much. First time always try to apply half rate until you figure out your spreader/fertilizer. A handheld scotts Wizz is great for low qty stuff.