A Comparative Eye
Those who prefer to work in Sight-Size understand that it is not always possible to do so. Therefore, they’ll resort to comparative measurement when necessary. If they’re well-trained in Sight-Size they would have no difficulty in doing a drawing to whatever scale desired using comparative measurement. Then again, scaling your vision is a distinct skill so it only makes sense to specifically train it. The exercises presented in A Comparative Eye will do precisely that.
2 PDF files: A Comparative Eye Course Book, and A Comparative Eye Workbook