Answering only part of your question re: recommendation of terry cloth with gray tone - I believe this is to find out if your Zone III exposure of the terry cloth (i.e. 2 stops below meter reading of Zone V) results in a proper Print III value when exposed on your chosen paper at the reference standard printing time. That is, the print should show full detail & should represent the "full detail in shadow" that is the accepted definition of a Zone III exposure. However, it is not essential that the terry cloth be gray, as the brighter/darker ambient light conditions might make a lighter/darker terry cloth necessary to accommodate the available shutter & speed combinations to achieve the total number of tones you want to test for.(posted 8279 days ago)