Train Your Comparative Eye
Although some may prefer to always work in Sight-Size, not all do or are able to all the time. Furthermore, not every subject or scene lends itself to the Sight-Size arrangement. In those cases the only other option is comparative measurement. Scaling your drawing should be no trouble for you if you’re skilled in seeing through Sight-Size. Since it is a distinct skill, however, it only makes sense to train that ability. And once you do, you’ll have gained a comparative eye.