about print_disksize

Hi All£¬

In print_disksize(target.c)
mb = ((size / 512) - (((size / 512) / 625) - 974)) / 1950; 

Why this? 
512¡¢625¡¢974¡¢1950 mean what?

Why not like this,
Mb = size / (1000 * 1000)?Simpler and easier to understand

Thanks£¬
Ruifeng Yang