I just found this Bellows Factor Conversion method. lens focal length of 210mm and a bellows draw of 450mm. Pretend these numbers are f-stops...drop the zeroes and you have 21 and 45. How many stops between 21 (f22) and 45 (f45)... Right, the bellows factor is 2 stops. Increase your exposure by 2 stops. If this is a valid method it is really simple. What do you experienced folks think?