It is a series number used by Yamaha for one of the thrones in their drum throne series. Thrones with a number higher than the 640 will most likely be a more robust or better style of throne.
Numbers like these are usually only a quicker reference used by manufacturers to the description of a product.
640
60% of 640= 60% * 640= 0.6 * 640= 384
48% of 640 = 48% * 640 = 0.48 * 640 = 307.2
85% of 640= 85% * 640= 0.85 * 640= 544
30% of 640 = 30% * 640 = 0.3 * 640 = 192
110% of 640= 110% * 640= 1.1 * 640= 704
640
1920
640
640
45% of 640 =0.45*640 =288
640