The FW pattern and regular frame checkering pattern are an identical three point pattern; so, if you have one, use it for reference .
The measurement at the centerline is approximately 4 3/8" from point to point with the first point 1" from where the wood makes contact with the forend iron.( border not included.
As with the FW pattern the panel extends to 1/4" from the upper edge of the forearm, again border not included. Any other questions or clarification, just ask