beginners fitting tool question

I fitted a function to some data I collected using the cftool. I want now to know what is the x value for a given y, but I cannot find how to do this.I saw the data cursor in the tools for the plot generated by the GUI, but it's not accurate, and beside I want eventually to automate the process so that I could easily repeat it for different data sets. Can anyone explain or just refer me to the right document to read?