Wow, been awhile. It sounds as if you need a 5 stage ring counter.
(pausing remembering just how to do it)

Using 5 J K flip flops you hook all the Ts together and in a ring (thus
the name) you tie the J to the Q and the K to the not Q of the previous
flip flop.Output at any of the five phases can be tapped at any of the
Q terminals for a 20% on time, or the not Q for 80% on time.

How far in the cowbwebs of my mind did I have to go ? The last time I
thought about ring counters was when I was builing a bootleg cable
decoder for the old SSAVI2 scrambling method.

There are also programmable dividers if you want a one chip solution.
With 7 terminals per FF, you only can get two in a 16 pin DIP so this
means three chips minimum. If you go with the ring counter you don't
use the other two, C and cD I think, you should consult the datasheet
for whatever chips you use to see whether you should ground them or
leave them float.

To sum it up, digital division is the only way to get reliable
operation. If you wind up with three dual flip flops you can also use
the last one to get on times other than 20% if desired.

Good luck with it.

JURB