From Draftsperson.net
Reduction ratios for ISO sheets & scales. A useful list to keep near the photocopier.
 A3 to A4 = ~70%
 A2 to A3 = ~70%
 A2 to A4 = 50%
 A1 to A2 = ~70%
 A1 to A3 = 50%
 A1 to A4 = ~35%
 A0 to A1 = ~70%
 A0 to A2 = 50%
 A0 to A3 = ~35%
 A0 to A4 = 25%

 B1 to A1 = ~82%
 B1 to B2 = ~70%
 B1 to A2 = ~60%
 B1 to B3 = 50%
 B1 to A3 = ~40%
 B1 to B4 = ~35%
 B1 to A4 = ~28%

 1:1000 to 1:500 = 200%
 1:500 to 1:250 = 200%
 1:250 to 1:200 = 125%
 1:200 to 1:100 = 200%
 1:100 to 1:50 = 200%
