Reading values from a logarithmic scaled x-axis

Hey, I'm having trouble reading off values from a couple of charts for a molecular biology lab. I'm supposed to get the X value for each of the four lines at 50% of the max. response (i.e. where a line drawn from 50 on the y axis would intercept the lines I've plotted). Log scales are hard to work with though, is there any way I can get excel to do this for me? I've attached a screen grab of them to show what I'm talking about. Thanks to anyone who can help.