If you are using a ruler marked only in inches then the best sub-division of an onch which approximates to 5 millimetres would be one-fifth of an inch. There are 25.4 millimetres in 1 inch.
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On a ruler, 4.5 mm is slightly less than half a centimeter. It is located just past the 4 mm mark and before the 5 mm mark. The distance is roughly the thickness of a credit card or a thin hair. If using a metric ruler, it will be clearly marked, usually with a shorter line than the 5 mm mark.
1 cm = 10 mm. Start with ruler at 0 and mark ribbon at 30 cm. Then place ruler at the mark and measure 30 cm more and make another mark- you are up to 60 cm or 600 mm. Then place ruler at this mark and measure 5 cm, or 50 mm. Now you have a 650 mm ribbon.
To measure the mm of an item you would need to use a metric ruler. The metric ruler has cm and mm on it.
20 mm should still be 20 mm!
With considerable difficulty! With a 30 cm ruler you can easily ascertain that the length is between 2 and 3 ruler-lengths: 600 mm and 900 mm. Maybe you can judge that the the remainder, after 600 mm is less than half of the 30 cm ruler. So that tells you that it is between 600 mm and 750 mm.