Problem with Flats

Simon I sent you an image of the PTC via email.

The spreadsheet is no-more beautiful than the Crisp spreadsheet and would confuse more than help, I feel, as
I still have queries about some of the workings and until I'm confident to explain any part of it.

There are one or two "does one really need to do that - what does that really do anyway? - if you know what I mean.
I'm not ignoring Crisp's spreadsheet - just want my own understanding to be improved first.

This is a subject that one or two of us bump off every now and again - there will be updates.

Regards
Mark C.

Found a really good document that explains the PTC process and what data to put where, even when your camera does not have an "Overscan region"

www.narrowbandimaging.com/incoming/flats_a_to_z.pdf