Perhaps you have decided that the company has adequate reserves based on what you have calculated they have spent over the last couple of years. The problem with your calculation is that it ignores the far greater expenses of preparing clinical trial material and running of number of clinical studies. This process is very expensive and prta is going to need to have their war chest full..
The fact that they are gearing up for clinical trials is a positive sign. However, there is no guarantee that they will be successful.
I don't find a public offering particularly troubling for a biotech company. Sometimes the money is needed to conduct costly clinical trials in the future. I personally feel that is a sign that PRTA has a number of good candidates and that they will need additional money to fund these projects.