Can anyone tell me from where on a large format lens (or any 'thick lens' for that matter) the lens plane is measured from? If I have a 90mm Super Angulon mounted on a lens board, where is the lens plane? ie, from where will it be 90mm (nominally) to the film plane when focussed at infinity?

The possible answers I can think of are:
-The closest bit of glass to the film
-The front of the lens board
-The rear of the lens board
-Some other point nominated by the lens manufacturer.

Note: If I had a 90mm Super Angulon I may be able to work this out. Needless to say, I don't.

All help appreciated.

John Ford.