Calculating cost changes needed to maintain margins

Hi there,
I have a price book for our products and currently use the following:

A1 - 2018 Cost
B1 - 2018 Retail
C1 - 2018 Margin % calculation as follows =1-(A1/B1)

I am adding the following columns:
D1 - 2019 Cost
E1 - 2019 Retail
F1 - 2019 Margin % calculation as follows =1-(D1/E1)

Our cost prices have gone up and our retail prices will change so I need to have a final calculation that tells me how much we would need to reduce our cost prices by to maintain the current 2018 margin.
It's not as simple as just working out the difference between the two cost prices as our retail prices will change in 2019...

Any help greatly appreciated!

Many thanks

Hi Wayne,

Thanks for that. I think I managed to get it to work using the following:
=2019 retail-(2019 retail * 2018 margin)

Seems to have done the trick, however having checked, your formula does the same thing.
More than one way to do it.

All the best