18/6 is a good light schedule for autos. I start my seedlings ( up to week 2 ) at 20/4 then drop to 18/6 thru end of grow.
12/12 " CAN " be used once it is flowering ...
For Example : you have space available in your Flowering Tent ( with regular photo plants ) running on their 12/12 cycle. BUT because most Autos have a reduced VEG STAGE , it is important to get as much growth and size during that time BEFORE flowering. Autos continue to stretch during flowering also , so i would feed GROW NUTES UNTIL stretch stops then switch to BLOOM NUTES.