Progress Bar with 5% steps

I have the following simple code:

import sys
num_tasks = 1000000
for i in range(num_tasks):
    if ((i/(num_tasks-1)) % 5):
        sys.stderr.write('\rdone {0:.0%}'.format(i/(num_tasks-1)))

My problem is that i just want to show the 5% steps like:

done 5%, done 10%, done 15%,...

What am i doing wrong?