Internal Standard Curve in Sample


I have a method that requires an internal standard curve to be generated from the samples where the first sample is endogenous but the next 3 injections are the sample spiked at a 20%, 50% then 100% of the standard concentration and I was trying to figure out how to have a curve be generated off those levels so a line of best fit is generated and determine the relative concentration in the endogenous sample.  Has anyone here had a similar method to work without the need to have a custom field?