roy, to find a base time, mount the plate on the camera and measure the distance to the film plane. Ideally you need to know the diameter of your pinhole but I'm guessing it will be close to 0.3 mm. Divide the focal length (say it's about 40mm) by the pinhole diameter (40mm/0.3mm) and you end up with f133. f133 is not quite 1/3 stop more than f128 so I'd work on adding 1/2 stop over the f128 number I come up with. So, assuming Sunny 16 conditions, 1/60 @ f16 for your Acros (In my experiments, it's not an 100asa film), you'd need to add 6 1/2 stops (16-22-32-45-64-90-128+1/2) which gives a shutter speed of 1.5 sec (1/60-1/30-1/15-1/8-1/4-1/2-1sec-1 1/2sec) I'd shoot 1 and 2 sec brackets. Easy to compute than write down!