Divide 12 (inches) by the scale:
12 divided by 24 = 0.5 inches. Half an inch equals one foot.
12 divided by 20.3 = 0.5911330049. Call it 0.6
0.6 inches equals a foot.
Standard rulers are divided by 8ths, not 10ths, so you cant find 0.6 neatly on a standard ruler. (Its between 4/8 and 5/8) you can buy rulers divided by tenths of an inch, but you would have to seek one out.
Or, an easier measurent when using a standard ruler: 15mm equals a foot. A centimeter and a half.