Fit the data using nonlinear regression to this equation. You want to determine the slope and the X intercept, with SE and confidence interval. Nonlinear regression can fit two line segments to different portions of the data, so that the two meet at a defined X value. See Example 3. Two linear regression segments that meet at the breakpoint .
Prism then fits a model to the data.
If you start from a graph, Prism fits to the data plotted on that graph. To fit a straight line on a semilog plot requires use of nonlinear regression. If the Y-axis is logarithmic use this equation. Y-axis is equivalent to taking the antilog. So this equation appears linear when graphed on a logarithmic Y-axis.