So the final equation of line becomes y = mx + b:
(%Response-Median) = 63.7495854(LOG Dose) + (-59.85831)
So to solve for the value of LOG Dose fo which we have 50% response we set y=0 above to get:
0 = mx + b, or x = -b/m
x = 0.93896861
But x is LOG Dose so to get Dose we must take 10 to the power of x:
10 to power x = 8.68897627
This says that at a dose of 8.7 there will be 50% response.