Thank you Bill Burk for your answer!

anyway, i am more confused now...because i haven't understood how to correctly meter and set the flash...also taking into consideration the inconsistency in the output of the flash,....i would use a sb800

Having said that i am wondering if it is not easier to follow Ralph's advice, getting a TP 4x5 with 21 bars/31bars, taping it flat to a window and masking it to reduce flare.
I could then use an extension tube (36mm) on my medium format camera with an 80 mm lens (set to infinity) so that the 4x5 stouffer fits onto a 6x6 negative (finally adding 1 stop to exposure because of the extension).
So far so good...

But how do i meter the correct exposure then/and where (meaning which bar number) on the stouffer?

I have got a Pentax Digital Spotmeter but i am not sure if i can meter so narrow, meaning 1 single bar on the tablet, because after asking at Stouffer's they told me that: The step size of the TP4x5-21 is 9.525mm and the TP4x5-31 is 6.4mm.

Any advice?!

Thank you for your help!

Best regards, Christoph.