My current plan is extremely inconsistent with my actual data. For instance, the project started a year late and has already slipped by almost 15 months.  Using my original plan, the actual defect data yields a defect tuning factor of 2000% when I run the Defect Tuning Calculator function. 

Is the plan being factored into the defect tuning calculations? If so, can significant variation between the plan and the actuals result in an incorrect tuning factor?

When looking for the best tuning factor, SLIM-Control must make some assumptions in order to generate a set of theoretical defect curves and find the one that best fits your actual data. If these assumptions are incorrect (for example, if either the expected phase 3 duration or expected effort are wrong) then any calculations based on these assumptions will be affected. 

Before running the Defect Tuning Calculator, it’s a good idea to review the current plan to ensure reasonable consistency with your actual data. If your actuals have diverged significantly from the current plan, it should be updated before defect tuning is performed. To update the current plan, run a forecast, make the forecast the current plan, then re-run the Defect Tuning Calculator.