Seikomatics are some of my favourite watches from 1960's. Yours looks like a typically lovely example but I am not sure that you have a 6218. The dial suggests it's a 6206 but if the rear says different then it may be redial. On the issue of quicksetting the day, your method is fine and there should be no adverse consequences. If you want to read more on the 62 series 1960's Seikomatics, then feel free to have a nose around my blog here. I've also written about the GS you mentioned in your opening paragraph here and here.
Martin