Hi there,
Hopefully the measurements that I took before I started ripping my car to pieces will help you. The measurements below were taken from the NS sill, but you can just reverse for the OS. The measurements were taken from an original unmodified sill.

The image below shows where I took the measuremets from.
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The following are the actual measurements
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These should be useful to you as they show the increments of the rear sill flare