Scale (proportional) dividers
These are a wonderful tool for helping you achieve accurate proportions in your initial drawing. Once you have set the central pivot to the desired position, you can take measurements from your reference using the tips at one end of the dividers and then use the resulting distance at the opposite end to ensure that you achieve an accurate enlargement/reduction in your drawing.
There are many different styles of design available. The two that I have shown above are the ones that I use.
You can download details of how to use these dividers on the Help Sheets page