A more accurate way to figure this out would be to lower the car until the rocker is at the height you want, then put the spindle at the correct height from the ground. Spindle height should match your rolling radius.
A 3" drop is very subjective. Most people do not start with their cars at stock height as they will have settled over time. Springs compress, fenders sag, different numbers of shims are used to get panel alignment, etc.
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
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