That Works but not for me

I need to explain my problem more fully I have a formula that i repeat many times down, in cell a1 i have '=C1 * d1 /e1'

this formula is copied down 1000's times, however the value of C1 & D1 change intermittently
I want to automate the process the value of c1 & d1 will remain the same for 20 rows and then change to new values for a further 50 rows and this continues.

At the moment I have in cell C1 '=$F$1' & D1 '=$G$1' which is the first 20 rows then I manually change the value C1 to =$F$2 & D1 =$G$2 for 50 rows and so on

Again Be gentle and thank you for your help