Error bars - ratio of averages

#1
Hi
I was just wondering if anyone could help me.
I have 4 values for growth rates of a yeast with no treatment, and 4 values of the growth rate with treatment. I can plot the averages as bar graphs and use the standard error of the mean as the error bars.

But, I also want to show the % change in growth rate (so one divided by the other, X100) - how do I go about putting error bars on this charts? Do I sum the errors of the 2 original averages? or do something funky with them?

Any help would be greatly appreciated.
Thanks

Sara